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1========================================
2A description of what robust futexes are
3========================================
4
5:Started by: Ingo Molnar <mingo@redhat.com>
6
7Background
8----------
9
10what are robust futexes? To answer that, we first need to understand
11what futexes are: normal futexes are special types of locks that in the
12noncontended case can be acquired/released from userspace without having
13to enter the kernel.
14
15A futex is in essence a user-space address, e.g. a 32-bit lock variable
16field. If userspace notices contention (the lock is already owned and
17someone else wants to grab it too) then the lock is marked with a value
18that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT)
19syscall is used to wait for the other guy to release it. The kernel
20creates a 'futex queue' internally, so that it can later on match up the
21waiter with the waker - without them having to know about each other.
22When the owner thread releases the futex, it notices (via the variable
23value) that there were waiter(s) pending, and does the
24sys_futex(FUTEX_WAKE) syscall to wake them up.  Once all waiters have
25taken and released the lock, the futex is again back to 'uncontended'
26state, and there's no in-kernel state associated with it. The kernel
27completely forgets that there ever was a futex at that address. This
28method makes futexes very lightweight and scalable.
29
30"Robustness" is about dealing with crashes while holding a lock: if a
31process exits prematurely while holding a pthread_mutex_t lock that is
32also shared with some other process (e.g. yum segfaults while holding a
33pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need
34to be notified that the last owner of the lock exited in some irregular
35way.
36
37To solve such types of problems, "robust mutex" userspace APIs were
38created: pthread_mutex_lock() returns an error value if the owner exits
39prematurely - and the new owner can decide whether the data protected by
40the lock can be recovered safely.
41
42There is a big conceptual problem with futex based mutexes though: it is
43the kernel that destroys the owner task (e.g. due to a SEGFAULT), but
44the kernel cannot help with the cleanup: if there is no 'futex queue'
45(and in most cases there is none, futexes being fast lightweight locks)
46then the kernel has no information to clean up after the held lock!
47Userspace has no chance to clean up after the lock either - userspace is
48the one that crashes, so it has no opportunity to clean up. Catch-22.
49
50In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot
51is needed to release that futex based lock. This is one of the leading
52bugreports against yum.
53
54To solve this problem, the traditional approach was to extend the vma
55(virtual memory area descriptor) concept to have a notion of 'pending
56robust futexes attached to this area'. This approach requires 3 new
57syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
58FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
59they have a robust_head set. This approach has two fundamental problems
60left:
61
62 - it has quite complex locking and race scenarios. The vma-based
63   approach had been pending for years, but they are still not completely
64   reliable.
65
66 - they have to scan _every_ vma at sys_exit() time, per thread!
67
68The second disadvantage is a real killer: pthread_exit() takes around 1
69microsecond on Linux, but with thousands (or tens of thousands) of vmas
70every pthread_exit() takes a millisecond or more, also totally
71destroying the CPU's L1 and L2 caches!
72
73This is very much noticeable even for normal process sys_exit_group()
74calls: the kernel has to do the vma scanning unconditionally! (this is
75because the kernel has no knowledge about how many robust futexes there
76are to be cleaned up, because a robust futex might have been registered
77in another task, and the futex variable might have been simply mmap()-ed
78into this process's address space).
79
80This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that
81normal kernels can turn it off, but worse than that: the overhead makes
82robust futexes impractical for any type of generic Linux distribution.
83
84So something had to be done.
85
86New approach to robust futexes
87------------------------------
88
89At the heart of this new approach there is a per-thread private list of
90robust locks that userspace is holding (maintained by glibc) - which
91userspace list is registered with the kernel via a new syscall [this
92registration happens at most once per thread lifetime]. At do_exit()
93time, the kernel checks this user-space list: are there any robust futex
94locks to be cleaned up?
95
96In the common case, at do_exit() time, there is no list registered, so
97the cost of robust futexes is just a simple current->robust_list != NULL
98comparison. If the thread has registered a list, then normally the list
99is empty. If the thread/process crashed or terminated in some incorrect
100way then the list might be non-empty: in this case the kernel carefully
101walks the list [not trusting it], and marks all locks that are owned by
102this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if
103any).
104
105The list is guaranteed to be private and per-thread at do_exit() time,
106so it can be accessed by the kernel in a lockless way.
107
108There is one race possible though: since adding to and removing from the
109list is done after the futex is acquired by glibc, there is a few
110instructions window for the thread (or process) to die there, leaving
111the futex hung. To protect against this possibility, userspace (glibc)
112also maintains a simple per-thread 'list_op_pending' field, to allow the
113kernel to clean up if the thread dies after acquiring the lock, but just
114before it could have added itself to the list. Glibc sets this
115list_op_pending field before it tries to acquire the futex, and clears
116it after the list-add (or list-remove) has finished.
117
118That's all that is needed - all the rest of robust-futex cleanup is done
119in userspace [just like with the previous patches].
120
121Ulrich Drepper has implemented the necessary glibc support for this new
122mechanism, which fully enables robust mutexes.
123
124Key differences of this userspace-list based approach, compared to the
125vma based method:
126
127 - it's much, much faster: at thread exit time, there's no need to loop
128   over every vma (!), which the VM-based method has to do. Only a very
129   simple 'is the list empty' op is done.
130
131 - no VM changes are needed - 'struct address_space' is left alone.
132
133 - no registration of individual locks is needed: robust mutexes don't
134   need any extra per-lock syscalls. Robust mutexes thus become a very
135   lightweight primitive - so they don't force the application designer
136   to do a hard choice between performance and robustness - robust
137   mutexes are just as fast.
138
139 - no per-lock kernel allocation happens.
140
141 - no resource limits are needed.
142
143 - no kernel-space recovery call (FUTEX_RECOVER) is needed.
144
145 - the implementation and the locking is "obvious", and there are no
146   interactions with the VM.
147
148Performance
149-----------
150
151I have benchmarked the time needed for the kernel to process a list of 1
152million (!) held locks, using the new method [on a 2GHz CPU]:
153
154 - with FUTEX_WAIT set [contended mutex]: 130 msecs
155 - without FUTEX_WAIT set [uncontended mutex]: 30 msecs
156
157I have also measured an approach where glibc does the lock notification
158[which it currently does for !pshared robust mutexes], and that took 256
159msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls
160userspace had to do.
161
162(1 million held locks are unheard of - we expect at most a handful of
163locks to be held at a time. Nevertheless it's nice to know that this
164approach scales nicely.)
165
166Implementation details
167----------------------
168
169The patch adds two new syscalls: one to register the userspace list, and
170one to query the registered list pointer::
171
172 asmlinkage long
173 sys_set_robust_list(struct robust_list_head __user *head,
174                     size_t len);
175
176 asmlinkage long
177 sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
178                     size_t __user *len_ptr);
179
180List registration is very fast: the pointer is simply stored in
181current->robust_list. [Note that in the future, if robust futexes become
182widespread, we could extend sys_clone() to register a robust-list head
183for new threads, without the need of another syscall.]
184
185So there is virtually zero overhead for tasks not using robust futexes,
186and even for robust futex users, there is only one extra syscall per
187thread lifetime, and the cleanup operation, if it happens, is fast and
188straightforward. The kernel doesn't have any internal distinction between
189robust and normal futexes.
190
191If a futex is found to be held at exit time, the kernel sets the
192following bit of the futex word::
193
194	#define FUTEX_OWNER_DIED        0x40000000
195
196and wakes up the next futex waiter (if any). User-space does the rest of
197the cleanup.
198
199Otherwise, robust futexes are acquired by glibc by putting the TID into
200the futex field atomically. Waiters set the FUTEX_WAITERS bit::
201
202	#define FUTEX_WAITERS           0x80000000
203
204and the remaining bits are for the TID.
205
206Testing, architecture support
207-----------------------------
208
209I've tested the new syscalls on x86 and x86_64, and have made sure the
210parsing of the userspace list is robust [ ;-) ] even if the list is
211deliberately corrupted.
212
213i386 and x86_64 syscalls are wired up at the moment, and Ulrich has
214tested the new glibc code (on x86_64 and i386), and it works for his
215robust-mutex testcases.
216
217All other architectures should build just fine too - but they won't have
218the new syscalls yet.
219
220Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
221inline function before writing up the syscalls.
222