1<?xml version="1.0"?> <!-- -*- sgml -*- --> 2<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" 3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" 4[ <!ENTITY % vg-entities SYSTEM "../../docs/xml/vg-entities.xml"> %vg-entities; ]> 5 6 7<chapter id="drd-manual" xreflabel="DRD: a thread error detector"> 8 <title>DRD: a thread error detector</title> 9 10<para>To use this tool, you must specify 11<option>--tool=drd</option> 12on the Valgrind command line.</para> 13 14 15<sect1 id="drd-manual.overview" xreflabel="Overview"> 16<title>Overview</title> 17 18<para> 19DRD is a Valgrind tool for detecting errors in multithreaded C and C++ 20programs. The tool works for any program that uses the POSIX threading 21primitives or that uses threading concepts built on top of the POSIX threading 22primitives. 23</para> 24 25<sect2 id="drd-manual.mt-progr-models" xreflabel="MT-progr-models"> 26<title>Multithreaded Programming Paradigms</title> 27 28<para> 29There are two possible reasons for using multithreading in a program: 30<itemizedlist> 31 <listitem> 32 <para> 33 To model concurrent activities. Assigning one thread to each activity 34 can be a great simplification compared to multiplexing the states of 35 multiple activities in a single thread. This is why most server software 36 and embedded software is multithreaded. 37 </para> 38 </listitem> 39 <listitem> 40 <para> 41 To use multiple CPU cores simultaneously for speeding up 42 computations. This is why many High Performance Computing (HPC) 43 applications are multithreaded. 44 </para> 45 </listitem> 46</itemizedlist> 47</para> 48 49<para> 50Multithreaded programs can use one or more of the following programming 51paradigms. Which paradigm is appropriate depends e.g. on the application type. 52Some examples of multithreaded programming paradigms are: 53<itemizedlist> 54 <listitem> 55 <para> 56 Locking. Data that is shared over threads is protected from concurrent 57 accesses via locking. E.g. the POSIX threads library, the Qt library 58 and the Boost.Thread library support this paradigm directly. 59 </para> 60 </listitem> 61 <listitem> 62 <para> 63 Message passing. No data is shared between threads, but threads exchange 64 data by passing messages to each other. Examples of implementations of 65 the message passing paradigm are MPI and CORBA. 66 </para> 67 </listitem> 68 <listitem> 69 <para> 70 Automatic parallelization. A compiler converts a sequential program into 71 a multithreaded program. The original program may or may not contain 72 parallelization hints. One example of such parallelization hints is the 73 OpenMP standard. In this standard a set of directives are defined which 74 tell a compiler how to parallelize a C, C++ or Fortran program. OpenMP 75 is well suited for computational intensive applications. As an example, 76 an open source image processing software package is using OpenMP to 77 maximize performance on systems with multiple CPU 78 cores. GCC supports the 79 OpenMP standard from version 4.2.0 on. 80 </para> 81 </listitem> 82 <listitem> 83 <para> 84 Software Transactional Memory (STM). Any data that is shared between 85 threads is updated via transactions. After each transaction it is 86 verified whether there were any conflicting transactions. If there were 87 conflicts, the transaction is aborted, otherwise it is committed. This 88 is a so-called optimistic approach. There is a prototype of the Intel C++ 89 Compiler available that supports STM. Research about the addition of 90 STM support to GCC is ongoing. 91 </para> 92 </listitem> 93</itemizedlist> 94</para> 95 96<para> 97DRD supports any combination of multithreaded programming paradigms as 98long as the implementation of these paradigms is based on the POSIX 99threads primitives. DRD however does not support programs that use 100e.g. Linux' futexes directly. Attempts to analyze such programs with 101DRD will cause DRD to report many false positives. 102</para> 103 104</sect2> 105 106 107<sect2 id="drd-manual.pthreads-model" xreflabel="Pthreads-model"> 108<title>POSIX Threads Programming Model</title> 109 110<para> 111POSIX threads, also known as Pthreads, is the most widely available 112threading library on Unix systems. 113</para> 114 115<para> 116The POSIX threads programming model is based on the following abstractions: 117<itemizedlist> 118 <listitem> 119 <para> 120 A shared address space. All threads running within the same 121 process share the same address space. All data, whether shared or 122 not, is identified by its address. 123 </para> 124 </listitem> 125 <listitem> 126 <para> 127 Regular load and store operations, which allow to read values 128 from or to write values to the memory shared by all threads 129 running in the same process. 130 </para> 131 </listitem> 132 <listitem> 133 <para> 134 Atomic store and load-modify-store operations. While these are 135 not mentioned in the POSIX threads standard, most 136 microprocessors support atomic memory operations. 137 </para> 138 </listitem> 139 <listitem> 140 <para> 141 Threads. Each thread represents a concurrent activity. 142 </para> 143 </listitem> 144 <listitem> 145 <para> 146 Synchronization objects and operations on these synchronization 147 objects. The following types of synchronization objects have been 148 defined in the POSIX threads standard: mutexes, condition variables, 149 semaphores, reader-writer synchronization objects, barriers and 150 spinlocks. 151 </para> 152 </listitem> 153</itemizedlist> 154</para> 155 156<para> 157Which source code statements generate which memory accesses depends on 158the <emphasis>memory model</emphasis> of the programming language being 159used. There is not yet a definitive memory model for the C and C++ 160languages. For a draft memory model, see also the document 161<ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html"> 162WG21/N2338: Concurrency memory model compiler consequences</ulink>. 163</para> 164 165<para> 166For more information about POSIX threads, see also the Single UNIX 167Specification version 3, also known as 168<ulink url="http://www.opengroup.org/onlinepubs/000095399/idx/threads.html"> 169IEEE Std 1003.1</ulink>. 170</para> 171 172</sect2> 173 174 175<sect2 id="drd-manual.mt-problems" xreflabel="MT-Problems"> 176<title>Multithreaded Programming Problems</title> 177 178<para> 179Depending on which multithreading paradigm is being used in a program, 180one or more of the following problems can occur: 181<itemizedlist> 182 <listitem> 183 <para> 184 Data races. One or more threads access the same memory location without 185 sufficient locking. Most but not all data races are programming errors 186 and are the cause of subtle and hard-to-find bugs. 187 </para> 188 </listitem> 189 <listitem> 190 <para> 191 Lock contention. One thread blocks the progress of one or more other 192 threads by holding a lock too long. 193 </para> 194 </listitem> 195 <listitem> 196 <para> 197 Improper use of the POSIX threads API. Most implementations of the POSIX 198 threads API have been optimized for runtime speed. Such implementations 199 will not complain on certain errors, e.g. when a mutex is being unlocked 200 by another thread than the thread that obtained a lock on the mutex. 201 </para> 202 </listitem> 203 <listitem> 204 <para> 205 Deadlock. A deadlock occurs when two or more threads wait for 206 each other indefinitely. 207 </para> 208 </listitem> 209 <listitem> 210 <para> 211 False sharing. If threads that run on different processor cores 212 access different variables located in the same cache line 213 frequently, this will slow down the involved threads a lot due 214 to frequent exchange of cache lines. 215 </para> 216 </listitem> 217</itemizedlist> 218</para> 219 220<para> 221Although the likelihood of the occurrence of data races can be reduced 222through a disciplined programming style, a tool for automatic 223detection of data races is a necessity when developing multithreaded 224software. DRD can detect these, as well as lock contention and 225improper use of the POSIX threads API. 226</para> 227 228</sect2> 229 230 231<sect2 id="drd-manual.data-race-detection" xreflabel="data-race-detection"> 232<title>Data Race Detection</title> 233 234<para> 235The result of load and store operations performed by a multithreaded program 236depends on the order in which memory operations are performed. This order is 237determined by: 238<orderedlist> 239 <listitem> 240 <para> 241 All memory operations performed by the same thread are performed in 242 <emphasis>program order</emphasis>, that is, the order determined by the 243 program source code and the results of previous load operations. 244 </para> 245 </listitem> 246 <listitem> 247 <para> 248 Synchronization operations determine certain ordering constraints on 249 memory operations performed by different threads. These ordering 250 constraints are called the <emphasis>synchronization order</emphasis>. 251 </para> 252 </listitem> 253</orderedlist> 254The combination of program order and synchronization order is called the 255<emphasis>happens-before relationship</emphasis>. This concept was first 256defined by S. Adve et al in the paper <emphasis>Detecting data races on weak 257memory systems</emphasis>, ACM SIGARCH Computer Architecture News, v.19 n.3, 258p.234-243, May 1991. 259</para> 260 261<para> 262Two memory operations <emphasis>conflict</emphasis> if both operations are 263performed by different threads, refer to the same memory location and at least 264one of them is a store operation. 265</para> 266 267<para> 268A multithreaded program is <emphasis>data-race free</emphasis> if all 269conflicting memory accesses are ordered by synchronization 270operations. 271</para> 272 273<para> 274A well known way to ensure that a multithreaded program is data-race 275free is to ensure that a locking discipline is followed. It is e.g. 276possible to associate a mutex with each shared data item, and to hold 277a lock on the associated mutex while the shared data is accessed. 278</para> 279 280<para> 281All programs that follow a locking discipline are data-race free, but not all 282data-race free programs follow a locking discipline. There exist multithreaded 283programs where access to shared data is arbitrated via condition variables, 284semaphores or barriers. As an example, a certain class of HPC applications 285consists of a sequence of computation steps separated in time by barriers, and 286where these barriers are the only means of synchronization. Although there are 287many conflicting memory accesses in such applications and although such 288applications do not make use mutexes, most of these applications do not 289contain data races. 290</para> 291 292<para> 293There exist two different approaches for verifying the correctness of 294multithreaded programs at runtime. The approach of the so-called Eraser 295algorithm is to verify whether all shared memory accesses follow a consistent 296locking strategy. And the happens-before data race detectors verify directly 297whether all interthread memory accesses are ordered by synchronization 298operations. While the last approach is more complex to implement, and while it 299is more sensitive to OS scheduling, it is a general approach that works for 300all classes of multithreaded programs. An important advantage of 301happens-before data race detectors is that these do not report any false 302positives. 303</para> 304 305<para> 306DRD is based on the happens-before algorithm. 307</para> 308 309</sect2> 310 311 312</sect1> 313 314 315<sect1 id="drd-manual.using-drd" xreflabel="Using DRD"> 316<title>Using DRD</title> 317 318<sect2 id="drd-manual.options" xreflabel="DRD Command-line Options"> 319<title>DRD Command-line Options</title> 320 321<para>The following command-line options are available for controlling the 322behavior of the DRD tool itself:</para> 323 324<!-- start of xi:include in the manpage --> 325<variablelist id="drd.opts.list"> 326 <varlistentry> 327 <term> 328 <option><![CDATA[--check-stack-var=<yes|no> [default: no]]]></option> 329 </term> 330 <listitem> 331 <para> 332 Controls whether DRD detects data races on stack 333 variables. Verifying stack variables is disabled by default because 334 most programs do not share stack variables over threads. 335 </para> 336 </listitem> 337 </varlistentry> 338 <varlistentry> 339 <term> 340 <option><![CDATA[--exclusive-threshold=<n> [default: off]]]></option> 341 </term> 342 <listitem> 343 <para> 344 Print an error message if any mutex or writer lock has been 345 held longer than the time specified in milliseconds. This 346 option enables the detection of lock contention. 347 </para> 348 </listitem> 349 </varlistentry> 350 <varlistentry> 351 <term> 352 <option><![CDATA[--join-list-vol=<n> [default: 10]]]></option> 353 </term> 354 <listitem> 355 <para> 356 Data races that occur between a statement at the end of one thread 357 and another thread can be missed if memory access information is 358 discarded immediately after a thread has been joined. This option 359 allows to specify for how many joined threads memory access information 360 should be retained. 361 </para> 362 </listitem> 363 </varlistentry> 364 <varlistentry> 365 <term> 366 <option> 367 <![CDATA[--first-race-only=<yes|no> [default: no]]]> 368 </option> 369 </term> 370 <listitem> 371 <para> 372 Whether to report only the first data race that has been detected on a 373 memory location or all data races that have been detected on a memory 374 location. 375 </para> 376 </listitem> 377 </varlistentry> 378 <varlistentry> 379 <term> 380 <option> 381 <![CDATA[--free-is-write=<yes|no> [default: no]]]> 382 </option> 383 </term> 384 <listitem> 385 <para> 386 Whether to report races between accessing memory and freeing 387 memory. Enabling this option may cause DRD to run slightly 388 slower. Notes:</para> 389 <itemizedlist> 390 <listitem> 391 <para> 392 Don't enable this option when using custom memory allocators 393 that use 394 the <computeroutput>VG_USERREQ__MALLOCLIKE_BLOCK</computeroutput> 395 and <computeroutput>VG_USERREQ__FREELIKE_BLOCK</computeroutput> 396 because that would result in false positives. 397 </para> 398 </listitem> 399 <listitem> 400 <para>Don't enable this option when using reference-counted 401 objects because that will result in false positives, even when 402 that code has been annotated properly with 403 <computeroutput>ANNOTATE_HAPPENS_BEFORE</computeroutput> 404 and <computeroutput>ANNOTATE_HAPPENS_AFTER</computeroutput>. See 405 e.g. the output of the following command for an example: 406 <computeroutput>valgrind --tool=drd --free-is-write=yes 407 drd/tests/annotate_smart_pointer</computeroutput>. 408 </para> 409 </listitem> 410 </itemizedlist> 411 </listitem> 412 </varlistentry> 413 <varlistentry> 414 <term> 415 <option> 416 <![CDATA[--report-signal-unlocked=<yes|no> [default: yes]]]> 417 </option> 418 </term> 419 <listitem> 420 <para> 421 Whether to report calls to 422 <function>pthread_cond_signal</function> and 423 <function>pthread_cond_broadcast</function> where the mutex 424 associated with the signal through 425 <function>pthread_cond_wait</function> or 426 <function>pthread_cond_timed_wait</function>is not locked at 427 the time the signal is sent. Sending a signal without holding 428 a lock on the associated mutex is a common programming error 429 which can cause subtle race conditions and unpredictable 430 behavior. There exist some uncommon synchronization patterns 431 however where it is safe to send a signal without holding a 432 lock on the associated mutex. 433 </para> 434 </listitem> 435 </varlistentry> 436 <varlistentry> 437 <term> 438 <option><![CDATA[--segment-merging=<yes|no> [default: yes]]]></option> 439 </term> 440 <listitem> 441 <para> 442 Controls segment merging. Segment merging is an algorithm to 443 limit memory usage of the data race detection 444 algorithm. Disabling segment merging may improve the accuracy 445 of the so-called 'other segments' displayed in race reports 446 but can also trigger an out of memory error. 447 </para> 448 </listitem> 449 </varlistentry> 450 <varlistentry> 451 <term> 452 <option><![CDATA[--segment-merging-interval=<n> [default: 10]]]></option> 453 </term> 454 <listitem> 455 <para> 456 Perform segment merging only after the specified number of new 457 segments have been created. This is an advanced configuration option 458 that allows to choose whether to minimize DRD's memory usage by 459 choosing a low value or to let DRD run faster by choosing a slightly 460 higher value. The optimal value for this parameter depends on the 461 program being analyzed. The default value works well for most programs. 462 </para> 463 </listitem> 464 </varlistentry> 465 <varlistentry> 466 <term> 467 <option><![CDATA[--shared-threshold=<n> [default: off]]]></option> 468 </term> 469 <listitem> 470 <para> 471 Print an error message if a reader lock has been held longer 472 than the specified time (in milliseconds). This option enables 473 the detection of lock contention. 474 </para> 475 </listitem> 476 </varlistentry> 477 <varlistentry> 478 <term> 479 <option><![CDATA[--show-confl-seg=<yes|no> [default: yes]]]></option> 480 </term> 481 <listitem> 482 <para> 483 Show conflicting segments in race reports. Since this 484 information can help to find the cause of a data race, this 485 option is enabled by default. Disabling this option makes the 486 output of DRD more compact. 487 </para> 488 </listitem> 489 </varlistentry> 490 <varlistentry> 491 <term> 492 <option><![CDATA[--show-stack-usage=<yes|no> [default: no]]]></option> 493 </term> 494 <listitem> 495 <para> 496 Print stack usage at thread exit time. When a program creates a large 497 number of threads it becomes important to limit the amount of virtual 498 memory allocated for thread stacks. This option makes it possible to 499 observe how much stack memory has been used by each thread of the 500 client program. Note: the DRD tool itself allocates some temporary 501 data on the client thread stack. The space necessary for this 502 temporary data must be allocated by the client program when it 503 allocates stack memory, but is not included in stack usage reported by 504 DRD. 505 </para> 506 </listitem> 507 </varlistentry> 508</variablelist> 509<!-- end of xi:include in the manpage --> 510 511<!-- start of xi:include in the manpage --> 512<para> 513The following options are available for monitoring the behavior of the 514client program: 515</para> 516 517<variablelist id="drd.debugopts.list"> 518 <varlistentry> 519 <term> 520 <option><![CDATA[--trace-addr=<address> [default: none]]]></option> 521 </term> 522 <listitem> 523 <para> 524 Trace all load and store activity for the specified 525 address. This option may be specified more than once. 526 </para> 527 </listitem> 528 </varlistentry> 529 <varlistentry> 530 <term> 531 <option><![CDATA[--ptrace-addr=<address> [default: none]]]></option> 532 </term> 533 <listitem> 534 <para> 535 Trace all load and store activity for the specified address and keep 536 doing that even after the memory at that address has been freed and 537 reallocated. 538 </para> 539 </listitem> 540 </varlistentry> 541 <varlistentry> 542 <term> 543 <option><![CDATA[--trace-alloc=<yes|no> [default: no]]]></option> 544 </term> 545 <listitem> 546 <para> 547 Trace all memory allocations and deallocations. May produce a huge 548 amount of output. 549 </para> 550 </listitem> 551 </varlistentry> 552 <varlistentry> 553 <term> 554 <option><![CDATA[--trace-barrier=<yes|no> [default: no]]]></option> 555 </term> 556 <listitem> 557 <para> 558 Trace all barrier activity. 559 </para> 560 </listitem> 561 </varlistentry> 562 <varlistentry> 563 <term> 564 <option><![CDATA[--trace-cond=<yes|no> [default: no]]]></option> 565 </term> 566 <listitem> 567 <para> 568 Trace all condition variable activity. 569 </para> 570 </listitem> 571 </varlistentry> 572 <varlistentry> 573 <term> 574 <option><![CDATA[--trace-fork-join=<yes|no> [default: no]]]></option> 575 </term> 576 <listitem> 577 <para> 578 Trace all thread creation and all thread termination events. 579 </para> 580 </listitem> 581 </varlistentry> 582 <varlistentry> 583 <term> 584 <option><![CDATA[--trace-hb=<yes|no> [default: no]]]></option> 585 </term> 586 <listitem> 587 <para> 588 Trace execution of the <literal>ANNOTATE_HAPPENS_BEFORE()</literal>, 589 <literal>ANNOTATE_HAPPENS_AFTER()</literal> and 590 <literal>ANNOTATE_HAPPENS_DONE()</literal> client requests. 591 </para> 592 </listitem> 593 </varlistentry> 594 <varlistentry> 595 <term> 596 <option><![CDATA[--trace-mutex=<yes|no> [default: no]]]></option> 597 </term> 598 <listitem> 599 <para> 600 Trace all mutex activity. 601 </para> 602 </listitem> 603 </varlistentry> 604 <varlistentry> 605 <term> 606 <option><![CDATA[--trace-rwlock=<yes|no> [default: no]]]></option> 607 </term> 608 <listitem> 609 <para> 610 Trace all reader-writer lock activity. 611 </para> 612 </listitem> 613 </varlistentry> 614 <varlistentry> 615 <term> 616 <option><![CDATA[--trace-semaphore=<yes|no> [default: no]]]></option> 617 </term> 618 <listitem> 619 <para> 620 Trace all semaphore activity. 621 </para> 622 </listitem> 623 </varlistentry> 624</variablelist> 625<!-- end of xi:include in the manpage --> 626 627</sect2> 628 629 630<sect2 id="drd-manual.data-races" xreflabel="Data Races"> 631<title>Detected Errors: Data Races</title> 632 633<para> 634DRD prints a message every time it detects a data race. Please keep 635the following in mind when interpreting DRD's output: 636<itemizedlist> 637 <listitem> 638 <para> 639 Every thread is assigned a <emphasis>thread ID</emphasis> by the DRD 640 tool. A thread ID is a number. Thread ID's start at one and are never 641 recycled. 642 </para> 643 </listitem> 644 <listitem> 645 <para> 646 The term <emphasis>segment</emphasis> refers to a consecutive 647 sequence of load, store and synchronization operations, all 648 issued by the same thread. A segment always starts and ends at a 649 synchronization operation. Data race analysis is performed 650 between segments instead of between individual load and store 651 operations because of performance reasons. 652 </para> 653 </listitem> 654 <listitem> 655 <para> 656 There are always at least two memory accesses involved in a data 657 race. Memory accesses involved in a data race are called 658 <emphasis>conflicting memory accesses</emphasis>. DRD prints a 659 report for each memory access that conflicts with a past memory 660 access. 661 </para> 662 </listitem> 663</itemizedlist> 664</para> 665 666<para> 667Below you can find an example of a message printed by DRD when it 668detects a data race: 669</para> 670<programlisting><![CDATA[ 671$ valgrind --tool=drd --read-var-info=yes drd/tests/rwlock_race 672... 673==9466== Thread 3: 674==9466== Conflicting load by thread 3 at 0x006020b8 size 4 675==9466== at 0x400B6C: thread_func (rwlock_race.c:29) 676==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186) 677==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so) 678==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so) 679==9466== Location 0x6020b8 is 0 bytes inside local var "s_racy" 680==9466== declared at rwlock_race.c:18, in frame #0 of thread 3 681==9466== Other segment start (thread 2) 682==9466== at 0x4C2847D: pthread_rwlock_rdlock* (drd_pthread_intercepts.c:813) 683==9466== by 0x400B6B: thread_func (rwlock_race.c:28) 684==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186) 685==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so) 686==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so) 687==9466== Other segment end (thread 2) 688==9466== at 0x4C28B54: pthread_rwlock_unlock* (drd_pthread_intercepts.c:912) 689==9466== by 0x400B84: thread_func (rwlock_race.c:30) 690==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186) 691==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so) 692==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so) 693... 694]]></programlisting> 695 696<para> 697The above report has the following meaning: 698<itemizedlist> 699 <listitem> 700 <para> 701 The number in the column on the left is the process ID of the 702 process being analyzed by DRD. 703 </para> 704 </listitem> 705 <listitem> 706 <para> 707 The first line ("Thread 3") tells you the thread ID for 708 the thread in which context the data race has been detected. 709 </para> 710 </listitem> 711 <listitem> 712 <para> 713 The next line tells which kind of operation was performed (load or 714 store) and by which thread. On the same line the start address and the 715 number of bytes involved in the conflicting access are also displayed. 716 </para> 717 </listitem> 718 <listitem> 719 <para> 720 Next, the call stack of the conflicting access is displayed. If 721 your program has been compiled with debug information 722 (<option>-g</option>), this call stack will include file names and 723 line numbers. The two 724 bottommost frames in this call stack (<function>clone</function> 725 and <function>start_thread</function>) show how the NPTL starts 726 a thread. The third frame 727 (<function>vg_thread_wrapper</function>) is added by DRD. The 728 fourth frame (<function>thread_func</function>) is the first 729 interesting line because it shows the thread entry point, that 730 is the function that has been passed as the third argument to 731 <function>pthread_create</function>. 732 </para> 733 </listitem> 734 <listitem> 735 <para> 736 Next, the allocation context for the conflicting address is 737 displayed. For dynamically allocated data the allocation call 738 stack is shown. For static variables and stack variables the 739 allocation context is only shown when the option 740 <option>--read-var-info=yes</option> has been 741 specified. Otherwise DRD will print <computeroutput>Allocation 742 context: unknown</computeroutput>. 743 </para> 744 </listitem> 745 <listitem> 746 <para> 747 A conflicting access involves at least two memory accesses. For 748 one of these accesses an exact call stack is displayed, and for 749 the other accesses an approximate call stack is displayed, 750 namely the start and the end of the segments of the other 751 accesses. This information can be interpreted as follows: 752 <orderedlist> 753 <listitem> 754 <para> 755 Start at the bottom of both call stacks, and count the 756 number stack frames with identical function name, file 757 name and line number. In the above example the three 758 bottommost frames are identical 759 (<function>clone</function>, 760 <function>start_thread</function> and 761 <function>vg_thread_wrapper</function>). 762 </para> 763 </listitem> 764 <listitem> 765 <para> 766 The next higher stack frame in both call stacks now tells 767 you between in which source code region the other memory 768 access happened. The above output tells that the other 769 memory access involved in the data race happened between 770 source code lines 28 and 30 in file 771 <computeroutput>rwlock_race.c</computeroutput>. 772 </para> 773 </listitem> 774 </orderedlist> 775 </para> 776 </listitem> 777</itemizedlist> 778</para> 779 780</sect2> 781 782 783<sect2 id="drd-manual.lock-contention" xreflabel="Lock Contention"> 784<title>Detected Errors: Lock Contention</title> 785 786<para> 787Threads must be able to make progress without being blocked for too long by 788other threads. Sometimes a thread has to wait until a mutex or reader-writer 789synchronization object is unlocked by another thread. This is called 790<emphasis>lock contention</emphasis>. 791</para> 792 793<para> 794Lock contention causes delays. Such delays should be as short as 795possible. The two command line options 796<literal>--exclusive-threshold=<n></literal> and 797<literal>--shared-threshold=<n></literal> make it possible to 798detect excessive lock contention by making DRD report any lock that 799has been held longer than the specified threshold. An example: 800</para> 801<programlisting><![CDATA[ 802$ valgrind --tool=drd --exclusive-threshold=10 drd/tests/hold_lock -i 500 803... 804==10668== Acquired at: 805==10668== at 0x4C267C8: pthread_mutex_lock (drd_pthread_intercepts.c:395) 806==10668== by 0x400D92: main (hold_lock.c:51) 807==10668== Lock on mutex 0x7fefffd50 was held during 503 ms (threshold: 10 ms). 808==10668== at 0x4C26ADA: pthread_mutex_unlock (drd_pthread_intercepts.c:441) 809==10668== by 0x400DB5: main (hold_lock.c:55) 810... 811]]></programlisting> 812 813<para> 814The <literal>hold_lock</literal> test program holds a lock as long as 815specified by the <literal>-i</literal> (interval) argument. The DRD 816output reports that the lock acquired at line 51 in source file 817<literal>hold_lock.c</literal> and released at line 55 was held during 818503 ms, while a threshold of 10 ms was specified to DRD. 819</para> 820 821</sect2> 822 823 824<sect2 id="drd-manual.api-checks" xreflabel="API Checks"> 825<title>Detected Errors: Misuse of the POSIX threads API</title> 826 827<para> 828 DRD is able to detect and report the following misuses of the POSIX 829 threads API: 830 <itemizedlist> 831 <listitem> 832 <para> 833 Passing the address of one type of synchronization object 834 (e.g. a mutex) to a POSIX API call that expects a pointer to 835 another type of synchronization object (e.g. a condition 836 variable). 837 </para> 838 </listitem> 839 <listitem> 840 <para> 841 Attempts to unlock a mutex that has not been locked. 842 </para> 843 </listitem> 844 <listitem> 845 <para> 846 Attempts to unlock a mutex that was locked by another thread. 847 </para> 848 </listitem> 849 <listitem> 850 <para> 851 Attempts to lock a mutex of type 852 <literal>PTHREAD_MUTEX_NORMAL</literal> or a spinlock 853 recursively. 854 </para> 855 </listitem> 856 <listitem> 857 <para> 858 Destruction or deallocation of a locked mutex. 859 </para> 860 </listitem> 861 <listitem> 862 <para> 863 Sending a signal to a condition variable while no lock is held 864 on the mutex associated with the condition variable. 865 </para> 866 </listitem> 867 <listitem> 868 <para> 869 Calling <function>pthread_cond_wait</function> on a mutex 870 that is not locked, that is locked by another thread or that 871 has been locked recursively. 872 </para> 873 </listitem> 874 <listitem> 875 <para> 876 Associating two different mutexes with a condition variable 877 through <function>pthread_cond_wait</function>. 878 </para> 879 </listitem> 880 <listitem> 881 <para> 882 Destruction or deallocation of a condition variable that is 883 being waited upon. 884 </para> 885 </listitem> 886 <listitem> 887 <para> 888 Destruction or deallocation of a locked reader-writer synchronization 889 object. 890 </para> 891 </listitem> 892 <listitem> 893 <para> 894 Attempts to unlock a reader-writer synchronization object that was not 895 locked by the calling thread. 896 </para> 897 </listitem> 898 <listitem> 899 <para> 900 Attempts to recursively lock a reader-writer synchronization object 901 exclusively. 902 </para> 903 </listitem> 904 <listitem> 905 <para> 906 Attempts to pass the address of a user-defined reader-writer 907 synchronization object to a POSIX threads function. 908 </para> 909 </listitem> 910 <listitem> 911 <para> 912 Attempts to pass the address of a POSIX reader-writer synchronization 913 object to one of the annotations for user-defined reader-writer 914 synchronization objects. 915 </para> 916 </listitem> 917 <listitem> 918 <para> 919 Reinitialization of a mutex, condition variable, reader-writer 920 lock, semaphore or barrier. 921 </para> 922 </listitem> 923 <listitem> 924 <para> 925 Destruction or deallocation of a semaphore or barrier that is 926 being waited upon. 927 </para> 928 </listitem> 929 <listitem> 930 <para> 931 Missing synchronization between barrier wait and barrier destruction. 932 </para> 933 </listitem> 934 <listitem> 935 <para> 936 Exiting a thread without first unlocking the spinlocks, mutexes or 937 reader-writer synchronization objects that were locked by that thread. 938 </para> 939 </listitem> 940 <listitem> 941 <para> 942 Passing an invalid thread ID to <function>pthread_join</function> 943 or <function>pthread_cancel</function>. 944 </para> 945 </listitem> 946 </itemizedlist> 947</para> 948 949</sect2> 950 951 952<sect2 id="drd-manual.clientreqs" xreflabel="Client requests"> 953<title>Client Requests</title> 954 955<para> 956Just as for other Valgrind tools it is possible to let a client program 957interact with the DRD tool through client requests. In addition to the 958client requests several macros have been defined that allow to use the 959client requests in a convenient way. 960</para> 961 962<para> 963The interface between client programs and the DRD tool is defined in 964the header file <literal><valgrind/drd.h></literal>. The 965available macros and client requests are: 966<itemizedlist> 967 <listitem> 968 <para> 969 The macro <literal>DRD_GET_VALGRIND_THREADID</literal> and the 970 corresponding client 971 request <varname>VG_USERREQ__DRD_GET_VALGRIND_THREAD_ID</varname>. 972 Query the thread ID that has been assigned by the Valgrind core to the 973 thread executing this client request. Valgrind's thread ID's start at 974 one and are recycled in case a thread stops. 975 </para> 976 </listitem> 977 <listitem> 978 <para> 979 The macro <literal>DRD_GET_DRD_THREADID</literal> and the corresponding 980 client request <varname>VG_USERREQ__DRD_GET_DRD_THREAD_ID</varname>. 981 Query the thread ID that has been assigned by DRD to the thread 982 executing this client request. These are the thread ID's reported by DRD 983 in data race reports and in trace messages. DRD's thread ID's start at 984 one and are never recycled. 985 </para> 986 </listitem> 987 <listitem> 988 <para> 989 The macros <literal>DRD_IGNORE_VAR(x)</literal>, 990 <literal>ANNOTATE_TRACE_MEMORY(&x)</literal> and the corresponding 991 client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>. Some 992 applications contain intentional races. There exist e.g. applications 993 where the same value is assigned to a shared variable from two different 994 threads. It may be more convenient to suppress such races than to solve 995 these. This client request allows to suppress such races. 996 </para> 997 </listitem> 998 <listitem> 999 <para> 1000 The macro <literal>DRD_STOP_IGNORING_VAR(x)</literal> and the 1001 corresponding client request 1002 <varname>VG_USERREQ__DRD_FINISH_SUPPRESSION</varname>. Tell DRD 1003 to no longer ignore data races for the address range that was suppressed 1004 either via the macro <literal>DRD_IGNORE_VAR(x)</literal> or via the 1005 client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>. 1006 </para> 1007 </listitem> 1008 <listitem> 1009 <para> 1010 The macro <literal>DRD_TRACE_VAR(x)</literal>. Trace all load and store 1011 activity for the address range starting at <literal>&x</literal> and 1012 occupying <literal>sizeof(x)</literal> bytes. When DRD reports a data 1013 race on a specified variable, and it's not immediately clear which 1014 source code statements triggered the conflicting accesses, it can be 1015 very helpful to trace all activity on the offending memory location. 1016 </para> 1017 </listitem> 1018 <listitem> 1019 <para> 1020 The macro <literal>DRD_STOP_TRACING_VAR(x)</literal>. Stop tracing load 1021 and store activity for the address range starting 1022 at <literal>&x</literal> and occupying <literal>sizeof(x)</literal> 1023 bytes. 1024 </para> 1025 </listitem> 1026 <listitem> 1027 <para> 1028 The macro <literal>ANNOTATE_TRACE_MEMORY(&x)</literal>. Trace all 1029 load and store activity that touches at least the single byte at the 1030 address <literal>&x</literal>. 1031 </para> 1032 </listitem> 1033 <listitem> 1034 <para> 1035 The client request <varname>VG_USERREQ__DRD_START_TRACE_ADDR</varname>, 1036 which allows to trace all load and store activity for the specified 1037 address range. 1038 </para> 1039 </listitem> 1040 <listitem> 1041 <para> 1042 The client 1043 request <varname>VG_USERREQ__DRD_STOP_TRACE_ADDR</varname>. Do no longer 1044 trace load and store activity for the specified address range. 1045 </para> 1046 </listitem> 1047 <listitem> 1048 <para> 1049 The macro <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> tells DRD to 1050 insert a mark. Insert this macro just after an access to the variable at 1051 the specified address has been performed. 1052 </para> 1053 </listitem> 1054 <listitem> 1055 <para> 1056 The macro <literal>ANNOTATE_HAPPENS_AFTER(addr)</literal> tells DRD that 1057 the next access to the variable at the specified address should be 1058 considered to have happened after the access just before the latest 1059 <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> annotation that 1060 references the same variable. The purpose of these two macros is to tell 1061 DRD about the order of inter-thread memory accesses implemented via 1062 atomic memory operations. See 1063 also <literal>drd/tests/annotate_smart_pointer.cpp</literal> for an 1064 example. 1065 </para> 1066 </listitem> 1067 <listitem> 1068 <para> 1069 The macro <literal>ANNOTATE_RWLOCK_CREATE(rwlock)</literal> tells DRD 1070 that the object at address <literal>rwlock</literal> is a 1071 reader-writer synchronization object that is not a 1072 <literal>pthread_rwlock_t</literal> synchronization object. See 1073 also <literal>drd/tests/annotate_rwlock.c</literal> for an example. 1074 </para> 1075 </listitem> 1076 <listitem> 1077 <para> 1078 The macro <literal>ANNOTATE_RWLOCK_DESTROY(rwlock)</literal> tells DRD 1079 that the reader-writer synchronization object at 1080 address <literal>rwlock</literal> has been destroyed. 1081 </para> 1082 </listitem> 1083 <listitem> 1084 <para> 1085 The macro <literal>ANNOTATE_WRITERLOCK_ACQUIRED(rwlock)</literal> tells 1086 DRD that a writer lock has been acquired on the reader-writer 1087 synchronization object at address <literal>rwlock</literal>. 1088 </para> 1089 </listitem> 1090 <listitem> 1091 <para> 1092 The macro <literal>ANNOTATE_READERLOCK_ACQUIRED(rwlock)</literal> tells 1093 DRD that a reader lock has been acquired on the reader-writer 1094 synchronization object at address <literal>rwlock</literal>. 1095 </para> 1096 </listitem> 1097 <listitem> 1098 <para> 1099 The macro <literal>ANNOTATE_RWLOCK_ACQUIRED(rwlock, is_w)</literal> 1100 tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that 1101 a reader lock (when <literal>is_w == 0</literal>) has been acquired on 1102 the reader-writer synchronization object at 1103 address <literal>rwlock</literal>. 1104 </para> 1105 </listitem> 1106 <listitem> 1107 <para> 1108 The macro <literal>ANNOTATE_WRITERLOCK_RELEASED(rwlock)</literal> tells 1109 DRD that a writer lock has been released on the reader-writer 1110 synchronization object at address <literal>rwlock</literal>. 1111 </para> 1112 </listitem> 1113 <listitem> 1114 <para> 1115 The macro <literal>ANNOTATE_READERLOCK_RELEASED(rwlock)</literal> tells 1116 DRD that a reader lock has been released on the reader-writer 1117 synchronization object at address <literal>rwlock</literal>. 1118 </para> 1119 </listitem> 1120 <listitem> 1121 <para> 1122 The macro <literal>ANNOTATE_RWLOCK_RELEASED(rwlock, is_w)</literal> 1123 tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that 1124 a reader lock (when <literal>is_w == 0</literal>) has been released on 1125 the reader-writer synchronization object at 1126 address <literal>rwlock</literal>. 1127 </para> 1128 </listitem> 1129 <listitem> 1130 <para> 1131 The macro <literal>ANNOTATE_BARRIER_INIT(barrier, count, 1132 reinitialization_allowed)</literal> tells DRD that a new barrier object 1133 at the address <literal>barrier</literal> has been initialized, 1134 that <literal>count</literal> threads participate in each barrier and 1135 also whether or not barrier reinitialization without intervening 1136 destruction should be reported as an error. See 1137 also <literal>drd/tests/annotate_barrier.c</literal> for an example. 1138 </para> 1139 </listitem> 1140 <listitem> 1141 <para> 1142 The macro <literal>ANNOTATE_BARRIER_DESTROY(barrier)</literal> 1143 tells DRD that a barrier object is about to be destroyed. 1144 </para> 1145 </listitem> 1146 <listitem> 1147 <para> 1148 The macro <literal>ANNOTATE_BARRIER_WAIT_BEFORE(barrier)</literal> 1149 tells DRD that waiting for a barrier will start. 1150 </para> 1151 </listitem> 1152 <listitem> 1153 <para> 1154 The macro <literal>ANNOTATE_BARRIER_WAIT_AFTER(barrier)</literal> 1155 tells DRD that waiting for a barrier has finished. 1156 </para> 1157 </listitem> 1158 <listitem> 1159 <para> 1160 The macro <literal>ANNOTATE_BENIGN_RACE_SIZED(addr, size, 1161 descr)</literal> tells DRD that any races detected on the specified 1162 address are benign and hence should not be 1163 reported. The <literal>descr</literal> argument is ignored but can be 1164 used to document why data races on <literal>addr</literal> are benign. 1165 </para> 1166 </listitem> 1167 <listitem> 1168 <para> 1169 The macro <literal>ANNOTATE_BENIGN_RACE_STATIC(var, descr)</literal> 1170 tells DRD that any races detected on the specified static variable are 1171 benign and hence should not be reported. The <literal>descr</literal> 1172 argument is ignored but can be used to document why data races 1173 on <literal>var</literal> are benign. Note: this macro can only be 1174 used in C++ programs and not in C programs. 1175 </para> 1176 </listitem> 1177 <listitem> 1178 <para> 1179 The macro <literal>ANNOTATE_IGNORE_READS_BEGIN</literal> tells 1180 DRD to ignore all memory loads performed by the current thread. 1181 </para> 1182 </listitem> 1183 <listitem> 1184 <para> 1185 The macro <literal>ANNOTATE_IGNORE_READS_END</literal> tells 1186 DRD to stop ignoring the memory loads performed by the current thread. 1187 </para> 1188 </listitem> 1189 <listitem> 1190 <para> 1191 The macro <literal>ANNOTATE_IGNORE_WRITES_BEGIN</literal> tells 1192 DRD to ignore all memory stores performed by the current thread. 1193 </para> 1194 </listitem> 1195 <listitem> 1196 <para> 1197 The macro <literal>ANNOTATE_IGNORE_WRITES_END</literal> tells 1198 DRD to stop ignoring the memory stores performed by the current thread. 1199 </para> 1200 </listitem> 1201 <listitem> 1202 <para> 1203 The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN</literal> tells 1204 DRD to ignore all memory accesses performed by the current thread. 1205 </para> 1206 </listitem> 1207 <listitem> 1208 <para> 1209 The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_END</literal> tells 1210 DRD to stop ignoring the memory accesses performed by the current thread. 1211 </para> 1212 </listitem> 1213 <listitem> 1214 <para> 1215 The macro <literal>ANNOTATE_NEW_MEMORY(addr, size)</literal> tells 1216 DRD that the specified memory range has been allocated by a custom 1217 memory allocator in the client program and that the client program 1218 will start using this memory range. 1219 </para> 1220 </listitem> 1221 <listitem> 1222 <para> 1223 The macro <literal>ANNOTATE_THREAD_NAME(name)</literal> tells DRD to 1224 associate the specified name with the current thread and to include this 1225 name in the error messages printed by DRD. 1226 </para> 1227 </listitem> 1228 <listitem> 1229 <para> 1230 The macros <literal>VALGRIND_MALLOCLIKE_BLOCK</literal> and 1231 <literal>VALGRIND_FREELIKE_BLOCK</literal> from the Valgrind core are 1232 implemented; they are described in 1233 <xref linkend="manual-core-adv.clientreq"/>. 1234 </para> 1235 </listitem> 1236</itemizedlist> 1237</para> 1238 1239<para> 1240Note: if you compiled Valgrind yourself, the header file 1241<literal><valgrind/drd.h></literal> will have been installed in 1242the directory <literal>/usr/include</literal> by the command 1243<literal>make install</literal>. If you obtained Valgrind by 1244installing it as a package however, you will probably have to install 1245another package with a name like <literal>valgrind-devel</literal> 1246before Valgrind's header files are available. 1247</para> 1248 1249</sect2> 1250 1251 1252<sect2 id="drd-manual.gnome" xreflabel="GNOME"> 1253<title>Debugging GNOME Programs</title> 1254 1255<para> 1256GNOME applications use the threading primitives provided by the 1257<computeroutput>glib</computeroutput> and 1258<computeroutput>gthread</computeroutput> libraries. These libraries 1259are built on top of POSIX threads, and hence are directly supported by 1260DRD. Please keep in mind that you have to call 1261<function>g_thread_init</function> before creating any threads, or 1262DRD will report several data races on glib functions. See also the 1263<ulink 1264url="http://library.gnome.org/devel/glib/stable/glib-Threads.html">GLib 1265Reference Manual</ulink> for more information about 1266<function>g_thread_init</function>. 1267</para> 1268 1269<para> 1270One of the many facilities provided by the <literal>glib</literal> 1271library is a block allocator, called <literal>g_slice</literal>. You 1272have to disable this block allocator when using DRD by adding the 1273following to the shell environment variables: 1274<literal>G_SLICE=always-malloc</literal>. See also the <ulink 1275url="http://library.gnome.org/devel/glib/stable/glib-Memory-Slices.html">GLib 1276Reference Manual</ulink> for more information. 1277</para> 1278 1279</sect2> 1280 1281 1282<sect2 id="drd-manual.boost.thread" xreflabel="Boost.Thread"> 1283<title>Debugging Boost.Thread Programs</title> 1284 1285<para> 1286The Boost.Thread library is the threading library included with the 1287cross-platform Boost Libraries. This threading library is an early 1288implementation of the upcoming C++0x threading library. 1289</para> 1290 1291<para> 1292Applications that use the Boost.Thread library should run fine under DRD. 1293</para> 1294 1295<para> 1296More information about Boost.Thread can be found here: 1297<itemizedlist> 1298 <listitem> 1299 <para> 1300 Anthony Williams, <ulink 1301 url="http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html">Boost.Thread</ulink> 1302 Library Documentation, Boost website, 2007. 1303 </para> 1304 </listitem> 1305 <listitem> 1306 <para> 1307 Anthony Williams, <ulink 1308 url="http://www.ddj.com/cpp/211600441">What's New in Boost 1309 Threads?</ulink>, Recent changes to the Boost Thread library, 1310 Dr. Dobbs Magazine, October 2008. 1311 </para> 1312 </listitem> 1313</itemizedlist> 1314</para> 1315 1316</sect2> 1317 1318 1319<sect2 id="drd-manual.openmp" xreflabel="OpenMP"> 1320<title>Debugging OpenMP Programs</title> 1321 1322<para> 1323OpenMP stands for <emphasis>Open Multi-Processing</emphasis>. The OpenMP 1324standard consists of a set of compiler directives for C, C++ and Fortran 1325programs that allows a compiler to transform a sequential program into a 1326parallel program. OpenMP is well suited for HPC applications and allows to 1327work at a higher level compared to direct use of the POSIX threads API. While 1328OpenMP ensures that the POSIX API is used correctly, OpenMP programs can still 1329contain data races. So it definitely makes sense to verify OpenMP programs 1330with a thread checking tool. 1331</para> 1332 1333<para> 1334DRD supports OpenMP shared-memory programs generated by GCC. GCC 1335supports OpenMP since version 4.2.0. GCC's runtime support 1336for OpenMP programs is provided by a library called 1337<literal>libgomp</literal>. The synchronization primitives implemented 1338in this library use Linux' futex system call directly, unless the 1339library has been configured with the 1340<literal>--disable-linux-futex</literal> option. DRD only supports 1341libgomp libraries that have been configured with this option and in 1342which symbol information is present. For most Linux distributions this 1343means that you will have to recompile GCC. See also the script 1344<literal>drd/scripts/download-and-build-gcc</literal> in the 1345Valgrind source tree for an example of how to compile GCC. You will 1346also have to make sure that the newly compiled 1347<literal>libgomp.so</literal> library is loaded when OpenMP programs 1348are started. This is possible by adding a line similar to the 1349following to your shell startup script: 1350</para> 1351<programlisting><![CDATA[ 1352export LD_LIBRARY_PATH=~/gcc-4.4.0/lib64:~/gcc-4.4.0/lib: 1353]]></programlisting> 1354 1355<para> 1356As an example, the test OpenMP test program 1357<literal>drd/tests/omp_matinv</literal> triggers a data race 1358when the option -r has been specified on the command line. The data 1359race is triggered by the following code: 1360</para> 1361<programlisting><![CDATA[ 1362#pragma omp parallel for private(j) 1363for (j = 0; j < rows; j++) 1364{ 1365 if (i != j) 1366 { 1367 const elem_t factor = a[j * cols + i]; 1368 for (k = 0; k < cols; k++) 1369 { 1370 a[j * cols + k] -= a[i * cols + k] * factor; 1371 } 1372 } 1373} 1374]]></programlisting> 1375 1376<para> 1377The above code is racy because the variable <literal>k</literal> has 1378not been declared private. DRD will print the following error message 1379for the above code: 1380</para> 1381<programlisting><![CDATA[ 1382$ valgrind --tool=drd --check-stack-var=yes --read-var-info=yes drd/tests/omp_matinv 3 -t 2 -r 1383... 1384Conflicting store by thread 1/1 at 0x7fefffbc4 size 4 1385 at 0x4014A0: gj.omp_fn.0 (omp_matinv.c:203) 1386 by 0x401211: gj (omp_matinv.c:159) 1387 by 0x40166A: invert_matrix (omp_matinv.c:238) 1388 by 0x4019B4: main (omp_matinv.c:316) 1389Location 0x7fefffbc4 is 0 bytes inside local var "k" 1390declared at omp_matinv.c:160, in frame #0 of thread 1 1391... 1392]]></programlisting> 1393<para> 1394In the above output the function name <function>gj.omp_fn.0</function> 1395has been generated by GCC from the function name 1396<function>gj</function>. The allocation context information shows that the 1397data race has been caused by modifying the variable <literal>k</literal>. 1398</para> 1399 1400<para> 1401Note: for GCC versions before 4.4.0, no allocation context information is 1402shown. With these GCC versions the most usable information in the above output 1403is the source file name and the line number where the data race has been 1404detected (<literal>omp_matinv.c:203</literal>). 1405</para> 1406 1407<para> 1408For more information about OpenMP, see also 1409<ulink url="http://openmp.org/">openmp.org</ulink>. 1410</para> 1411 1412</sect2> 1413 1414 1415<sect2 id="drd-manual.cust-mem-alloc" xreflabel="Custom Memory Allocators"> 1416<title>DRD and Custom Memory Allocators</title> 1417 1418<para> 1419DRD tracks all memory allocation events that happen via the 1420standard memory allocation and deallocation functions 1421(<function>malloc</function>, <function>free</function>, 1422<function>new</function> and <function>delete</function>), via entry 1423and exit of stack frames or that have been annotated with Valgrind's 1424memory pool client requests. DRD uses memory allocation and deallocation 1425information for two purposes: 1426<itemizedlist> 1427 <listitem> 1428 <para> 1429 To know where the scope ends of POSIX objects that have not been 1430 destroyed explicitly. It is e.g. not required by the POSIX 1431 threads standard to call 1432 <function>pthread_mutex_destroy</function> before freeing the 1433 memory in which a mutex object resides. 1434 </para> 1435 </listitem> 1436 <listitem> 1437 <para> 1438 To know where the scope of variables ends. If e.g. heap memory 1439 has been used by one thread, that thread frees that memory, and 1440 another thread allocates and starts using that memory, no data 1441 races must be reported for that memory. 1442 </para> 1443 </listitem> 1444</itemizedlist> 1445</para> 1446 1447<para> 1448It is essential for correct operation of DRD that the tool knows about 1449memory allocation and deallocation events. When analyzing a client program 1450with DRD that uses a custom memory allocator, either instrument the custom 1451memory allocator with the <literal>VALGRIND_MALLOCLIKE_BLOCK</literal> 1452and <literal>VALGRIND_FREELIKE_BLOCK</literal> macros or disable the 1453custom memory allocator. 1454</para> 1455 1456<para> 1457As an example, the GNU libstdc++ library can be configured 1458to use standard memory allocation functions instead of memory pools by 1459setting the environment variable 1460<literal>GLIBCXX_FORCE_NEW</literal>. For more information, see also 1461the <ulink 1462url="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt04ch11.html">libstdc++ 1463manual</ulink>. 1464</para> 1465 1466</sect2> 1467 1468 1469<sect2 id="drd-manual.drd-versus-memcheck" xreflabel="DRD Versus Memcheck"> 1470<title>DRD Versus Memcheck</title> 1471 1472<para> 1473It is essential for correct operation of DRD that there are no memory 1474errors such as dangling pointers in the client program. Which means that 1475it is a good idea to make sure that your program is Memcheck-clean 1476before you analyze it with DRD. It is possible however that some of 1477the Memcheck reports are caused by data races. In this case it makes 1478sense to run DRD before Memcheck. 1479</para> 1480 1481<para> 1482So which tool should be run first? In case both DRD and Memcheck 1483complain about a program, a possible approach is to run both tools 1484alternatingly and to fix as many errors as possible after each run of 1485each tool until none of the two tools prints any more error messages. 1486</para> 1487 1488</sect2> 1489 1490 1491<sect2 id="drd-manual.resource-requirements" xreflabel="Resource Requirements"> 1492<title>Resource Requirements</title> 1493 1494<para> 1495The requirements of DRD with regard to heap and stack memory and the 1496effect on the execution time of client programs are as follows: 1497<itemizedlist> 1498 <listitem> 1499 <para> 1500 When running a program under DRD with default DRD options, 1501 between 1.1 and 3.6 times more memory will be needed compared to 1502 a native run of the client program. More memory will be needed 1503 if loading debug information has been enabled 1504 (<literal>--read-var-info=yes</literal>). 1505 </para> 1506 </listitem> 1507 <listitem> 1508 <para> 1509 DRD allocates some of its temporary data structures on the stack 1510 of the client program threads. This amount of data is limited to 1511 1 - 2 KB. Make sure that thread stacks are sufficiently large. 1512 </para> 1513 </listitem> 1514 <listitem> 1515 <para> 1516 Most applications will run between 20 and 50 times slower under 1517 DRD than a native single-threaded run. The slowdown will be most 1518 noticeable for applications which perform frequent mutex lock / 1519 unlock operations. 1520 </para> 1521 </listitem> 1522</itemizedlist> 1523</para> 1524 1525</sect2> 1526 1527 1528<sect2 id="drd-manual.effective-use" xreflabel="Effective Use"> 1529<title>Hints and Tips for Effective Use of DRD</title> 1530 1531<para> 1532The following information may be helpful when using DRD: 1533<itemizedlist> 1534 <listitem> 1535 <para> 1536 Make sure that debug information is present in the executable 1537 being analyzed, such that DRD can print function name and line 1538 number information in stack traces. Most compilers can be told 1539 to include debug information via compiler option 1540 <option>-g</option>. 1541 </para> 1542 </listitem> 1543 <listitem> 1544 <para> 1545 Compile with option <option>-O1</option> instead of 1546 <option>-O0</option>. This will reduce the amount of generated 1547 code, may reduce the amount of debug info and will speed up 1548 DRD's processing of the client program. For more information, 1549 see also <xref linkend="manual-core.started"/>. 1550 </para> 1551 </listitem> 1552 <listitem> 1553 <para> 1554 If DRD reports any errors on libraries that are part of your 1555 Linux distribution like e.g. <literal>libc.so</literal> or 1556 <literal>libstdc++.so</literal>, installing the debug packages 1557 for these libraries will make the output of DRD a lot more 1558 detailed. 1559 </para> 1560 </listitem> 1561 <listitem> 1562 <para> 1563 When using C++, do not send output from more than one thread to 1564 <literal>std::cout</literal>. Doing so would not only 1565 generate multiple data race reports, it could also result in 1566 output from several threads getting mixed up. Either use 1567 <function>printf</function> or do the following: 1568 <orderedlist> 1569 <listitem> 1570 <para>Derive a class from <literal>std::ostreambuf</literal> 1571 and let that class send output line by line to 1572 <literal>stdout</literal>. This will avoid that individual 1573 lines of text produced by different threads get mixed 1574 up.</para> 1575 </listitem> 1576 <listitem> 1577 <para>Create one instance of <literal>std::ostream</literal> 1578 for each thread. This makes stream formatting settings 1579 thread-local. Pass a per-thread instance of the class 1580 derived from <literal>std::ostreambuf</literal> to the 1581 constructor of each instance. </para> 1582 </listitem> 1583 <listitem> 1584 <para>Let each thread send its output to its own instance of 1585 <literal>std::ostream</literal> instead of 1586 <literal>std::cout</literal>.</para> 1587 </listitem> 1588 </orderedlist> 1589 </para> 1590 </listitem> 1591</itemizedlist> 1592</para> 1593 1594</sect2> 1595 1596 1597</sect1> 1598 1599 1600<sect1 id="drd-manual.Pthreads" xreflabel="Pthreads"> 1601<title>Using the POSIX Threads API Effectively</title> 1602 1603<sect2 id="drd-manual.mutex-types" xreflabel="mutex-types"> 1604<title>Mutex types</title> 1605 1606<para> 1607The Single UNIX Specification version two defines the following four 1608mutex types (see also the documentation of <ulink 1609url="http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_settype.html"><function>pthread_mutexattr_settype</function></ulink>): 1610<itemizedlist> 1611 <listitem> 1612 <para> 1613 <emphasis>normal</emphasis>, which means that no error checking 1614 is performed, and that the mutex is non-recursive. 1615 </para> 1616 </listitem> 1617 <listitem> 1618 <para> 1619 <emphasis>error checking</emphasis>, which means that the mutex 1620 is non-recursive and that error checking is performed. 1621 </para> 1622 </listitem> 1623 <listitem> 1624 <para> 1625 <emphasis>recursive</emphasis>, which means that a mutex may be 1626 locked recursively. 1627 </para> 1628 </listitem> 1629 <listitem> 1630 <para> 1631 <emphasis>default</emphasis>, which means that error checking 1632 behavior is undefined, and that the behavior for recursive 1633 locking is also undefined. Or: portable code must neither 1634 trigger error conditions through the Pthreads API nor attempt to 1635 lock a mutex of default type recursively. 1636 </para> 1637 </listitem> 1638</itemizedlist> 1639</para> 1640 1641<para> 1642In complex applications it is not always clear from beforehand which 1643mutex will be locked recursively and which mutex will not be locked 1644recursively. Attempts lock a non-recursive mutex recursively will 1645result in race conditions that are very hard to find without a thread 1646checking tool. So either use the error checking mutex type and 1647consistently check the return value of Pthread API mutex calls, or use 1648the recursive mutex type. 1649</para> 1650 1651</sect2> 1652 1653<sect2 id="drd-manual.condvar" xreflabel="condition-variables"> 1654<title>Condition variables</title> 1655 1656<para> 1657A condition variable allows one thread to wake up one or more other 1658threads. Condition variables are often used to notify one or more 1659threads about state changes of shared data. Unfortunately it is very 1660easy to introduce race conditions by using condition variables as the 1661only means of state information propagation. A better approach is to 1662let threads poll for changes of a state variable that is protected by 1663a mutex, and to use condition variables only as a thread wakeup 1664mechanism. See also the source file 1665<computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an 1666example of how to implement this concept in C++. The monitor concept 1667used in this example is a well known and very useful concept -- see 1668also Wikipedia for more information about the <ulink 1669url="http://en.wikipedia.org/wiki/Monitor_(synchronization)">monitor</ulink> 1670concept. 1671</para> 1672 1673</sect2> 1674 1675<sect2 id="drd-manual.pctw" xreflabel="pthread_cond_timedwait"> 1676<title>pthread_cond_timedwait and timeouts</title> 1677 1678<para> 1679Historically the function 1680<function>pthread_cond_timedwait</function> only allowed the 1681specification of an absolute timeout, that is a timeout independent of 1682the time when this function was called. However, almost every call to 1683this function expresses a relative timeout. This typically happens by 1684passing the sum of 1685<computeroutput>clock_gettime(CLOCK_REALTIME)</computeroutput> and a 1686relative timeout as the third argument. This approach is incorrect 1687since forward or backward clock adjustments by e.g. ntpd will affect 1688the timeout. A more reliable approach is as follows: 1689<itemizedlist> 1690 <listitem> 1691 <para> 1692 When initializing a condition variable through 1693 <function>pthread_cond_init</function>, specify that the timeout of 1694 <function>pthread_cond_timedwait</function> will use the clock 1695 <literal>CLOCK_MONOTONIC</literal> instead of 1696 <literal>CLOCK_REALTIME</literal>. You can do this via 1697 <computeroutput>pthread_condattr_setclock(..., 1698 CLOCK_MONOTONIC)</computeroutput>. 1699 </para> 1700 </listitem> 1701 <listitem> 1702 <para> 1703 When calling <function>pthread_cond_timedwait</function>, pass 1704 the sum of 1705 <computeroutput>clock_gettime(CLOCK_MONOTONIC)</computeroutput> 1706 and a relative timeout as the third argument. 1707 </para> 1708 </listitem> 1709</itemizedlist> 1710See also 1711<computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an 1712example. 1713</para> 1714 1715</sect2> 1716 1717</sect1> 1718 1719 1720<sect1 id="drd-manual.limitations" xreflabel="Limitations"> 1721<title>Limitations</title> 1722 1723<para>DRD currently has the following limitations:</para> 1724 1725<itemizedlist> 1726 <listitem> 1727 <para> 1728 DRD, just like Memcheck, will refuse to start on Linux 1729 distributions where all symbol information has been removed from 1730 <filename>ld.so</filename>. This is e.g. the case for the PPC editions 1731 of openSUSE and Gentoo. You will have to install the glibc debuginfo 1732 package on these platforms before you can use DRD. See also openSUSE 1733 bug <ulink url="http://bugzilla.novell.com/show_bug.cgi?id=396197"> 1734 396197</ulink> and Gentoo bug <ulink 1735 url="http://bugs.gentoo.org/214065">214065</ulink>. 1736 </para> 1737 </listitem> 1738 <listitem> 1739 <para> 1740 With gcc 4.4.3 and before, DRD may report data races on the C++ 1741 class <literal>std::string</literal> in a multithreaded program. This is 1742 a know <literal>libstdc++</literal> issue -- see also GCC bug 1743 <ulink url="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40518">40518</ulink> 1744 for more information. 1745 </para> 1746 </listitem> 1747 <listitem> 1748 <para> 1749 If you compile the DRD source code yourself, you need GCC 3.0 or 1750 later. GCC 2.95 is not supported. 1751 </para> 1752 </listitem> 1753 <listitem> 1754 <para> 1755 Of the two POSIX threads implementations for Linux, only the 1756 NPTL (Native POSIX Thread Library) is supported. The older 1757 LinuxThreads library is not supported. 1758 </para> 1759 </listitem> 1760</itemizedlist> 1761 1762</sect1> 1763 1764 1765<sect1 id="drd-manual.feedback" xreflabel="Feedback"> 1766<title>Feedback</title> 1767 1768<para> 1769If you have any comments, suggestions, feedback or bug reports about 1770DRD, feel free to either post a message on the Valgrind users mailing 1771list or to file a bug report. See also <ulink 1772url="&vg-url;">&vg-url;</ulink> for more information. 1773</para> 1774 1775</sect1> 1776 1777 1778</chapter> 1779