1.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.2-no-invariants-only 2 3=========================== 4Lockless Ring Buffer Design 5=========================== 6 7Copyright 2009 Red Hat Inc. 8 9:Author: Steven Rostedt <srostedt@redhat.com> 10:License: The GNU Free Documentation License, Version 1.2 11 (dual licensed under the GPL v2) 12:Reviewers: Mathieu Desnoyers, Huang Ying, Hidetoshi Seto, 13 and Frederic Weisbecker. 14 15 16Written for: 2.6.31 17 18Terminology used in this Document 19--------------------------------- 20 21tail 22 - where new writes happen in the ring buffer. 23 24head 25 - where new reads happen in the ring buffer. 26 27producer 28 - the task that writes into the ring buffer (same as writer) 29 30writer 31 - same as producer 32 33consumer 34 - the task that reads from the buffer (same as reader) 35 36reader 37 - same as consumer. 38 39reader_page 40 - A page outside the ring buffer used solely (for the most part) 41 by the reader. 42 43head_page 44 - a pointer to the page that the reader will use next 45 46tail_page 47 - a pointer to the page that will be written to next 48 49commit_page 50 - a pointer to the page with the last finished non-nested write. 51 52cmpxchg 53 - hardware-assisted atomic transaction that performs the following:: 54 55 A = B if previous A == C 56 57 R = cmpxchg(A, C, B) is saying that we replace A with B if and only 58 if current A is equal to C, and we put the old (current) 59 A into R 60 61 R gets the previous A regardless if A is updated with B or not. 62 63 To see if the update was successful a compare of ``R == C`` 64 may be used. 65 66The Generic Ring Buffer 67----------------------- 68 69The ring buffer can be used in either an overwrite mode or in 70producer/consumer mode. 71 72Producer/consumer mode is where if the producer were to fill up the 73buffer before the consumer could free up anything, the producer 74will stop writing to the buffer. This will lose most recent events. 75 76Overwrite mode is where if the producer were to fill up the buffer 77before the consumer could free up anything, the producer will 78overwrite the older data. This will lose the oldest events. 79 80No two writers can write at the same time (on the same per-cpu buffer), 81but a writer may interrupt another writer, but it must finish writing 82before the previous writer may continue. This is very important to the 83algorithm. The writers act like a "stack". The way interrupts works 84enforces this behavior:: 85 86 87 writer1 start 88 <preempted> writer2 start 89 <preempted> writer3 start 90 writer3 finishes 91 writer2 finishes 92 writer1 finishes 93 94This is very much like a writer being preempted by an interrupt and 95the interrupt doing a write as well. 96 97Readers can happen at any time. But no two readers may run at the 98same time, nor can a reader preempt/interrupt another reader. A reader 99cannot preempt/interrupt a writer, but it may read/consume from the 100buffer at the same time as a writer is writing, but the reader must be 101on another processor to do so. A reader may read on its own processor 102and can be preempted by a writer. 103 104A writer can preempt a reader, but a reader cannot preempt a writer. 105But a reader can read the buffer at the same time (on another processor) 106as a writer. 107 108The ring buffer is made up of a list of pages held together by a linked list. 109 110At initialization a reader page is allocated for the reader that is not 111part of the ring buffer. 112 113The head_page, tail_page and commit_page are all initialized to point 114to the same page. 115 116The reader page is initialized to have its next pointer pointing to 117the head page, and its previous pointer pointing to a page before 118the head page. 119 120The reader has its own page to use. At start up time, this page is 121allocated but is not attached to the list. When the reader wants 122to read from the buffer, if its page is empty (like it is on start-up), 123it will swap its page with the head_page. The old reader page will 124become part of the ring buffer and the head_page will be removed. 125The page after the inserted page (old reader_page) will become the 126new head page. 127 128Once the new page is given to the reader, the reader could do what 129it wants with it, as long as a writer has left that page. 130 131A sample of how the reader page is swapped: Note this does not 132show the head page in the buffer, it is for demonstrating a swap 133only. 134 135:: 136 137 +------+ 138 |reader| RING BUFFER 139 |page | 140 +------+ 141 +---+ +---+ +---+ 142 | |-->| |-->| | 143 | |<--| |<--| | 144 +---+ +---+ +---+ 145 ^ | ^ | 146 | +-------------+ | 147 +-----------------+ 148 149 150 +------+ 151 |reader| RING BUFFER 152 |page |-------------------+ 153 +------+ v 154 | +---+ +---+ +---+ 155 | | |-->| |-->| | 156 | | |<--| |<--| |<-+ 157 | +---+ +---+ +---+ | 158 | ^ | ^ | | 159 | | +-------------+ | | 160 | +-----------------+ | 161 +------------------------------------+ 162 163 +------+ 164 |reader| RING BUFFER 165 |page |-------------------+ 166 +------+ <---------------+ v 167 | ^ +---+ +---+ +---+ 168 | | | |-->| |-->| | 169 | | | | | |<--| |<-+ 170 | | +---+ +---+ +---+ | 171 | | | ^ | | 172 | | +-------------+ | | 173 | +-----------------------------+ | 174 +------------------------------------+ 175 176 +------+ 177 |buffer| RING BUFFER 178 |page |-------------------+ 179 +------+ <---------------+ v 180 | ^ +---+ +---+ +---+ 181 | | | | | |-->| | 182 | | New | | | |<--| |<-+ 183 | | Reader +---+ +---+ +---+ | 184 | | page ----^ | | 185 | | | | 186 | +-----------------------------+ | 187 +------------------------------------+ 188 189 190 191It is possible that the page swapped is the commit page and the tail page, 192if what is in the ring buffer is less than what is held in a buffer page. 193 194:: 195 196 reader page commit page tail page 197 | | | 198 v | | 199 +---+ | | 200 | |<----------+ | 201 | |<------------------------+ 202 | |------+ 203 +---+ | 204 | 205 v 206 +---+ +---+ +---+ +---+ 207 <---| |--->| |--->| |--->| |---> 208 --->| |<---| |<---| |<---| |<--- 209 +---+ +---+ +---+ +---+ 210 211This case is still valid for this algorithm. 212When the writer leaves the page, it simply goes into the ring buffer 213since the reader page still points to the next location in the ring 214buffer. 215 216 217The main pointers: 218 219 reader page 220 - The page used solely by the reader and is not part 221 of the ring buffer (may be swapped in) 222 223 head page 224 - the next page in the ring buffer that will be swapped 225 with the reader page. 226 227 tail page 228 - the page where the next write will take place. 229 230 commit page 231 - the page that last finished a write. 232 233The commit page only is updated by the outermost writer in the 234writer stack. A writer that preempts another writer will not move the 235commit page. 236 237When data is written into the ring buffer, a position is reserved 238in the ring buffer and passed back to the writer. When the writer 239is finished writing data into that position, it commits the write. 240 241Another write (or a read) may take place at anytime during this 242transaction. If another write happens it must finish before continuing 243with the previous write. 244 245 246 Write reserve:: 247 248 Buffer page 249 +---------+ 250 |written | 251 +---------+ <--- given back to writer (current commit) 252 |reserved | 253 +---------+ <--- tail pointer 254 | empty | 255 +---------+ 256 257 Write commit:: 258 259 Buffer page 260 +---------+ 261 |written | 262 +---------+ 263 |written | 264 +---------+ <--- next position for write (current commit) 265 | empty | 266 +---------+ 267 268 269 If a write happens after the first reserve:: 270 271 Buffer page 272 +---------+ 273 |written | 274 +---------+ <-- current commit 275 |reserved | 276 +---------+ <--- given back to second writer 277 |reserved | 278 +---------+ <--- tail pointer 279 280 After second writer commits:: 281 282 283 Buffer page 284 +---------+ 285 |written | 286 +---------+ <--(last full commit) 287 |reserved | 288 +---------+ 289 |pending | 290 |commit | 291 +---------+ <--- tail pointer 292 293 When the first writer commits:: 294 295 Buffer page 296 +---------+ 297 |written | 298 +---------+ 299 |written | 300 +---------+ 301 |written | 302 +---------+ <--(last full commit and tail pointer) 303 304 305The commit pointer points to the last write location that was 306committed without preempting another write. When a write that 307preempted another write is committed, it only becomes a pending commit 308and will not be a full commit until all writes have been committed. 309 310The commit page points to the page that has the last full commit. 311The tail page points to the page with the last write (before 312committing). 313 314The tail page is always equal to or after the commit page. It may 315be several pages ahead. If the tail page catches up to the commit 316page then no more writes may take place (regardless of the mode 317of the ring buffer: overwrite and produce/consumer). 318 319The order of pages is:: 320 321 head page 322 commit page 323 tail page 324 325Possible scenario:: 326 327 tail page 328 head page commit page | 329 | | | 330 v v v 331 +---+ +---+ +---+ +---+ 332 <---| |--->| |--->| |--->| |---> 333 --->| |<---| |<---| |<---| |<--- 334 +---+ +---+ +---+ +---+ 335 336There is a special case that the head page is after either the commit page 337and possibly the tail page. That is when the commit (and tail) page has been 338swapped with the reader page. This is because the head page is always 339part of the ring buffer, but the reader page is not. Whenever there 340has been less than a full page that has been committed inside the ring buffer, 341and a reader swaps out a page, it will be swapping out the commit page. 342 343:: 344 345 reader page commit page tail page 346 | | | 347 v | | 348 +---+ | | 349 | |<----------+ | 350 | |<------------------------+ 351 | |------+ 352 +---+ | 353 | 354 v 355 +---+ +---+ +---+ +---+ 356 <---| |--->| |--->| |--->| |---> 357 --->| |<---| |<---| |<---| |<--- 358 +---+ +---+ +---+ +---+ 359 ^ 360 | 361 head page 362 363 364In this case, the head page will not move when the tail and commit 365move back into the ring buffer. 366 367The reader cannot swap a page into the ring buffer if the commit page 368is still on that page. If the read meets the last commit (real commit 369not pending or reserved), then there is nothing more to read. 370The buffer is considered empty until another full commit finishes. 371 372When the tail meets the head page, if the buffer is in overwrite mode, 373the head page will be pushed ahead one. If the buffer is in producer/consumer 374mode, the write will fail. 375 376Overwrite mode:: 377 378 tail page 379 | 380 v 381 +---+ +---+ +---+ +---+ 382 <---| |--->| |--->| |--->| |---> 383 --->| |<---| |<---| |<---| |<--- 384 +---+ +---+ +---+ +---+ 385 ^ 386 | 387 head page 388 389 390 tail page 391 | 392 v 393 +---+ +---+ +---+ +---+ 394 <---| |--->| |--->| |--->| |---> 395 --->| |<---| |<---| |<---| |<--- 396 +---+ +---+ +---+ +---+ 397 ^ 398 | 399 head page 400 401 402 tail page 403 | 404 v 405 +---+ +---+ +---+ +---+ 406 <---| |--->| |--->| |--->| |---> 407 --->| |<---| |<---| |<---| |<--- 408 +---+ +---+ +---+ +---+ 409 ^ 410 | 411 head page 412 413Note, the reader page will still point to the previous head page. 414But when a swap takes place, it will use the most recent head page. 415 416 417Making the Ring Buffer Lockless: 418-------------------------------- 419 420The main idea behind the lockless algorithm is to combine the moving 421of the head_page pointer with the swapping of pages with the reader. 422State flags are placed inside the pointer to the page. To do this, 423each page must be aligned in memory by 4 bytes. This will allow the 2 424least significant bits of the address to be used as flags, since 425they will always be zero for the address. To get the address, 426simply mask out the flags:: 427 428 MASK = ~3 429 430 address & MASK 431 432Two flags will be kept by these two bits: 433 434 HEADER 435 - the page being pointed to is a head page 436 437 UPDATE 438 - the page being pointed to is being updated by a writer 439 and was or is about to be a head page. 440 441:: 442 443 reader page 444 | 445 v 446 +---+ 447 | |------+ 448 +---+ | 449 | 450 v 451 +---+ +---+ +---+ +---+ 452 <---| |--->| |-H->| |--->| |---> 453 --->| |<---| |<---| |<---| |<--- 454 +---+ +---+ +---+ +---+ 455 456 457The above pointer "-H->" would have the HEADER flag set. That is 458the next page is the next page to be swapped out by the reader. 459This pointer means the next page is the head page. 460 461When the tail page meets the head pointer, it will use cmpxchg to 462change the pointer to the UPDATE state:: 463 464 465 tail page 466 | 467 v 468 +---+ +---+ +---+ +---+ 469 <---| |--->| |-H->| |--->| |---> 470 --->| |<---| |<---| |<---| |<--- 471 +---+ +---+ +---+ +---+ 472 473 tail page 474 | 475 v 476 +---+ +---+ +---+ +---+ 477 <---| |--->| |-U->| |--->| |---> 478 --->| |<---| |<---| |<---| |<--- 479 +---+ +---+ +---+ +---+ 480 481"-U->" represents a pointer in the UPDATE state. 482 483Any access to the reader will need to take some sort of lock to serialize 484the readers. But the writers will never take a lock to write to the 485ring buffer. This means we only need to worry about a single reader, 486and writes only preempt in "stack" formation. 487 488When the reader tries to swap the page with the ring buffer, it 489will also use cmpxchg. If the flag bit in the pointer to the 490head page does not have the HEADER flag set, the compare will fail 491and the reader will need to look for the new head page and try again. 492Note, the flags UPDATE and HEADER are never set at the same time. 493 494The reader swaps the reader page as follows:: 495 496 +------+ 497 |reader| RING BUFFER 498 |page | 499 +------+ 500 +---+ +---+ +---+ 501 | |--->| |--->| | 502 | |<---| |<---| | 503 +---+ +---+ +---+ 504 ^ | ^ | 505 | +---------------+ | 506 +-----H-------------+ 507 508The reader sets the reader page next pointer as HEADER to the page after 509the head page:: 510 511 512 +------+ 513 |reader| RING BUFFER 514 |page |-------H-----------+ 515 +------+ v 516 | +---+ +---+ +---+ 517 | | |--->| |--->| | 518 | | |<---| |<---| |<-+ 519 | +---+ +---+ +---+ | 520 | ^ | ^ | | 521 | | +---------------+ | | 522 | +-----H-------------+ | 523 +--------------------------------------+ 524 525It does a cmpxchg with the pointer to the previous head page to make it 526point to the reader page. Note that the new pointer does not have the HEADER 527flag set. This action atomically moves the head page forward:: 528 529 +------+ 530 |reader| RING BUFFER 531 |page |-------H-----------+ 532 +------+ v 533 | ^ +---+ +---+ +---+ 534 | | | |-->| |-->| | 535 | | | |<--| |<--| |<-+ 536 | | +---+ +---+ +---+ | 537 | | | ^ | | 538 | | +-------------+ | | 539 | +-----------------------------+ | 540 +------------------------------------+ 541 542After the new head page is set, the previous pointer of the head page is 543updated to the reader page:: 544 545 +------+ 546 |reader| RING BUFFER 547 |page |-------H-----------+ 548 +------+ <---------------+ v 549 | ^ +---+ +---+ +---+ 550 | | | |-->| |-->| | 551 | | | | | |<--| |<-+ 552 | | +---+ +---+ +---+ | 553 | | | ^ | | 554 | | +-------------+ | | 555 | +-----------------------------+ | 556 +------------------------------------+ 557 558 +------+ 559 |buffer| RING BUFFER 560 |page |-------H-----------+ <--- New head page 561 +------+ <---------------+ v 562 | ^ +---+ +---+ +---+ 563 | | | | | |-->| | 564 | | New | | | |<--| |<-+ 565 | | Reader +---+ +---+ +---+ | 566 | | page ----^ | | 567 | | | | 568 | +-----------------------------+ | 569 +------------------------------------+ 570 571Another important point: The page that the reader page points back to 572by its previous pointer (the one that now points to the new head page) 573never points back to the reader page. That is because the reader page is 574not part of the ring buffer. Traversing the ring buffer via the next pointers 575will always stay in the ring buffer. Traversing the ring buffer via the 576prev pointers may not. 577 578Note, the way to determine a reader page is simply by examining the previous 579pointer of the page. If the next pointer of the previous page does not 580point back to the original page, then the original page is a reader page:: 581 582 583 +--------+ 584 | reader | next +----+ 585 | page |-------->| |<====== (buffer page) 586 +--------+ +----+ 587 | | ^ 588 | v | next 589 prev | +----+ 590 +------------->| | 591 +----+ 592 593The way the head page moves forward: 594 595When the tail page meets the head page and the buffer is in overwrite mode 596and more writes take place, the head page must be moved forward before the 597writer may move the tail page. The way this is done is that the writer 598performs a cmpxchg to convert the pointer to the head page from the HEADER 599flag to have the UPDATE flag set. Once this is done, the reader will 600not be able to swap the head page from the buffer, nor will it be able to 601move the head page, until the writer is finished with the move. 602 603This eliminates any races that the reader can have on the writer. The reader 604must spin, and this is why the reader cannot preempt the writer:: 605 606 tail page 607 | 608 v 609 +---+ +---+ +---+ +---+ 610 <---| |--->| |-H->| |--->| |---> 611 --->| |<---| |<---| |<---| |<--- 612 +---+ +---+ +---+ +---+ 613 614 tail page 615 | 616 v 617 +---+ +---+ +---+ +---+ 618 <---| |--->| |-U->| |--->| |---> 619 --->| |<---| |<---| |<---| |<--- 620 +---+ +---+ +---+ +---+ 621 622The following page will be made into the new head page:: 623 624 tail page 625 | 626 v 627 +---+ +---+ +---+ +---+ 628 <---| |--->| |-U->| |-H->| |---> 629 --->| |<---| |<---| |<---| |<--- 630 +---+ +---+ +---+ +---+ 631 632After the new head page has been set, we can set the old head page 633pointer back to NORMAL:: 634 635 tail page 636 | 637 v 638 +---+ +---+ +---+ +---+ 639 <---| |--->| |--->| |-H->| |---> 640 --->| |<---| |<---| |<---| |<--- 641 +---+ +---+ +---+ +---+ 642 643After the head page has been moved, the tail page may now move forward:: 644 645 tail page 646 | 647 v 648 +---+ +---+ +---+ +---+ 649 <---| |--->| |--->| |-H->| |---> 650 --->| |<---| |<---| |<---| |<--- 651 +---+ +---+ +---+ +---+ 652 653 654The above are the trivial updates. Now for the more complex scenarios. 655 656 657As stated before, if enough writes preempt the first write, the 658tail page may make it all the way around the buffer and meet the commit 659page. At this time, we must start dropping writes (usually with some kind 660of warning to the user). But what happens if the commit was still on the 661reader page? The commit page is not part of the ring buffer. The tail page 662must account for this:: 663 664 665 reader page commit page 666 | | 667 v | 668 +---+ | 669 | |<----------+ 670 | | 671 | |------+ 672 +---+ | 673 | 674 v 675 +---+ +---+ +---+ +---+ 676 <---| |--->| |-H->| |--->| |---> 677 --->| |<---| |<---| |<---| |<--- 678 +---+ +---+ +---+ +---+ 679 ^ 680 | 681 tail page 682 683If the tail page were to simply push the head page forward, the commit when 684leaving the reader page would not be pointing to the correct page. 685 686The solution to this is to test if the commit page is on the reader page 687before pushing the head page. If it is, then it can be assumed that the 688tail page wrapped the buffer, and we must drop new writes. 689 690This is not a race condition, because the commit page can only be moved 691by the outermost writer (the writer that was preempted). 692This means that the commit will not move while a writer is moving the 693tail page. The reader cannot swap the reader page if it is also being 694used as the commit page. The reader can simply check that the commit 695is off the reader page. Once the commit page leaves the reader page 696it will never go back on it unless a reader does another swap with the 697buffer page that is also the commit page. 698 699 700Nested writes 701------------- 702 703In the pushing forward of the tail page we must first push forward 704the head page if the head page is the next page. If the head page 705is not the next page, the tail page is simply updated with a cmpxchg. 706 707Only writers move the tail page. This must be done atomically to protect 708against nested writers:: 709 710 temp_page = tail_page 711 next_page = temp_page->next 712 cmpxchg(tail_page, temp_page, next_page) 713 714The above will update the tail page if it is still pointing to the expected 715page. If this fails, a nested write pushed it forward, the current write 716does not need to push it:: 717 718 719 temp page 720 | 721 v 722 tail page 723 | 724 v 725 +---+ +---+ +---+ +---+ 726 <---| |--->| |--->| |--->| |---> 727 --->| |<---| |<---| |<---| |<--- 728 +---+ +---+ +---+ +---+ 729 730Nested write comes in and moves the tail page forward:: 731 732 tail page (moved by nested writer) 733 temp page | 734 | | 735 v v 736 +---+ +---+ +---+ +---+ 737 <---| |--->| |--->| |--->| |---> 738 --->| |<---| |<---| |<---| |<--- 739 +---+ +---+ +---+ +---+ 740 741The above would fail the cmpxchg, but since the tail page has already 742been moved forward, the writer will just try again to reserve storage 743on the new tail page. 744 745But the moving of the head page is a bit more complex:: 746 747 tail page 748 | 749 v 750 +---+ +---+ +---+ +---+ 751 <---| |--->| |-H->| |--->| |---> 752 --->| |<---| |<---| |<---| |<--- 753 +---+ +---+ +---+ +---+ 754 755The write converts the head page pointer to UPDATE:: 756 757 tail page 758 | 759 v 760 +---+ +---+ +---+ +---+ 761 <---| |--->| |-U->| |--->| |---> 762 --->| |<---| |<---| |<---| |<--- 763 +---+ +---+ +---+ +---+ 764 765But if a nested writer preempts here, it will see that the next 766page is a head page, but it is also nested. It will detect that 767it is nested and will save that information. The detection is the 768fact that it sees the UPDATE flag instead of a HEADER or NORMAL 769pointer. 770 771The nested writer will set the new head page pointer:: 772 773 tail page 774 | 775 v 776 +---+ +---+ +---+ +---+ 777 <---| |--->| |-U->| |-H->| |---> 778 --->| |<---| |<---| |<---| |<--- 779 +---+ +---+ +---+ +---+ 780 781But it will not reset the update back to normal. Only the writer 782that converted a pointer from HEAD to UPDATE will convert it back 783to NORMAL:: 784 785 tail page 786 | 787 v 788 +---+ +---+ +---+ +---+ 789 <---| |--->| |-U->| |-H->| |---> 790 --->| |<---| |<---| |<---| |<--- 791 +---+ +---+ +---+ +---+ 792 793After the nested writer finishes, the outermost writer will convert 794the UPDATE pointer to NORMAL:: 795 796 797 tail page 798 | 799 v 800 +---+ +---+ +---+ +---+ 801 <---| |--->| |--->| |-H->| |---> 802 --->| |<---| |<---| |<---| |<--- 803 +---+ +---+ +---+ +---+ 804 805 806It can be even more complex if several nested writes came in and moved 807the tail page ahead several pages:: 808 809 810 (first writer) 811 812 tail page 813 | 814 v 815 +---+ +---+ +---+ +---+ 816 <---| |--->| |-H->| |--->| |---> 817 --->| |<---| |<---| |<---| |<--- 818 +---+ +---+ +---+ +---+ 819 820The write converts the head page pointer to UPDATE:: 821 822 tail page 823 | 824 v 825 +---+ +---+ +---+ +---+ 826 <---| |--->| |-U->| |--->| |---> 827 --->| |<---| |<---| |<---| |<--- 828 +---+ +---+ +---+ +---+ 829 830Next writer comes in, and sees the update and sets up the new 831head page:: 832 833 (second writer) 834 835 tail page 836 | 837 v 838 +---+ +---+ +---+ +---+ 839 <---| |--->| |-U->| |-H->| |---> 840 --->| |<---| |<---| |<---| |<--- 841 +---+ +---+ +---+ +---+ 842 843The nested writer moves the tail page forward. But does not set the old 844update page to NORMAL because it is not the outermost writer:: 845 846 tail page 847 | 848 v 849 +---+ +---+ +---+ +---+ 850 <---| |--->| |-U->| |-H->| |---> 851 --->| |<---| |<---| |<---| |<--- 852 +---+ +---+ +---+ +---+ 853 854Another writer preempts and sees the page after the tail page is a head page. 855It changes it from HEAD to UPDATE:: 856 857 (third writer) 858 859 tail page 860 | 861 v 862 +---+ +---+ +---+ +---+ 863 <---| |--->| |-U->| |-U->| |---> 864 --->| |<---| |<---| |<---| |<--- 865 +---+ +---+ +---+ +---+ 866 867The writer will move the head page forward:: 868 869 870 (third writer) 871 872 tail page 873 | 874 v 875 +---+ +---+ +---+ +---+ 876 <---| |--->| |-U->| |-U->| |-H-> 877 --->| |<---| |<---| |<---| |<--- 878 +---+ +---+ +---+ +---+ 879 880But now that the third writer did change the HEAD flag to UPDATE it 881will convert it to normal:: 882 883 884 (third writer) 885 886 tail page 887 | 888 v 889 +---+ +---+ +---+ +---+ 890 <---| |--->| |-U->| |--->| |-H-> 891 --->| |<---| |<---| |<---| |<--- 892 +---+ +---+ +---+ +---+ 893 894 895Then it will move the tail page, and return back to the second writer:: 896 897 898 (second writer) 899 900 tail page 901 | 902 v 903 +---+ +---+ +---+ +---+ 904 <---| |--->| |-U->| |--->| |-H-> 905 --->| |<---| |<---| |<---| |<--- 906 +---+ +---+ +---+ +---+ 907 908 909The second writer will fail to move the tail page because it was already 910moved, so it will try again and add its data to the new tail page. 911It will return to the first writer:: 912 913 914 (first writer) 915 916 tail page 917 | 918 v 919 +---+ +---+ +---+ +---+ 920 <---| |--->| |-U->| |--->| |-H-> 921 --->| |<---| |<---| |<---| |<--- 922 +---+ +---+ +---+ +---+ 923 924The first writer cannot know atomically if the tail page moved 925while it updates the HEAD page. It will then update the head page to 926what it thinks is the new head page:: 927 928 929 (first writer) 930 931 tail page 932 | 933 v 934 +---+ +---+ +---+ +---+ 935 <---| |--->| |-U->| |-H->| |-H-> 936 --->| |<---| |<---| |<---| |<--- 937 +---+ +---+ +---+ +---+ 938 939Since the cmpxchg returns the old value of the pointer the first writer 940will see it succeeded in updating the pointer from NORMAL to HEAD. 941But as we can see, this is not good enough. It must also check to see 942if the tail page is either where it use to be or on the next page:: 943 944 945 (first writer) 946 947 A B tail page 948 | | | 949 v v v 950 +---+ +---+ +---+ +---+ 951 <---| |--->| |-U->| |-H->| |-H-> 952 --->| |<---| |<---| |<---| |<--- 953 +---+ +---+ +---+ +---+ 954 955If tail page != A and tail page != B, then it must reset the pointer 956back to NORMAL. The fact that it only needs to worry about nested 957writers means that it only needs to check this after setting the HEAD page:: 958 959 960 (first writer) 961 962 A B tail page 963 | | | 964 v v v 965 +---+ +---+ +---+ +---+ 966 <---| |--->| |-U->| |--->| |-H-> 967 --->| |<---| |<---| |<---| |<--- 968 +---+ +---+ +---+ +---+ 969 970Now the writer can update the head page. This is also why the head page must 971remain in UPDATE and only reset by the outermost writer. This prevents 972the reader from seeing the incorrect head page:: 973 974 975 (first writer) 976 977 A B tail page 978 | | | 979 v v v 980 +---+ +---+ +---+ +---+ 981 <---| |--->| |--->| |--->| |-H-> 982 --->| |<---| |<---| |<---| |<--- 983 +---+ +---+ +---+ +---+ 984