linux/Documentation/robust-futexes.txt
<<
>>
Prefs
   1Started by: Ingo Molnar <mingo@redhat.com>
   2
   3Background
   4----------
   5
   6what are robust futexes? To answer that, we first need to understand
   7what futexes are: normal futexes are special types of locks that in the
   8noncontended case can be acquired/released from userspace without having
   9to enter the kernel.
  10
  11A futex is in essence a user-space address, e.g. a 32-bit lock variable
  12field. If userspace notices contention (the lock is already owned and
  13someone else wants to grab it too) then the lock is marked with a value
  14that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT)
  15syscall is used to wait for the other guy to release it. The kernel
  16creates a 'futex queue' internally, so that it can later on match up the
  17waiter with the waker - without them having to know about each other.
  18When the owner thread releases the futex, it notices (via the variable
  19value) that there were waiter(s) pending, and does the
  20sys_futex(FUTEX_WAKE) syscall to wake them up.  Once all waiters have
  21taken and released the lock, the futex is again back to 'uncontended'
  22state, and there's no in-kernel state associated with it. The kernel
  23completely forgets that there ever was a futex at that address. This
  24method makes futexes very lightweight and scalable.
  25
  26"Robustness" is about dealing with crashes while holding a lock: if a
  27process exits prematurely while holding a pthread_mutex_t lock that is
  28also shared with some other process (e.g. yum segfaults while holding a
  29pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need
  30to be notified that the last owner of the lock exited in some irregular
  31way.
  32
  33To solve such types of problems, "robust mutex" userspace APIs were
  34created: pthread_mutex_lock() returns an error value if the owner exits
  35prematurely - and the new owner can decide whether the data protected by
  36the lock can be recovered safely.
  37
  38There is a big conceptual problem with futex based mutexes though: it is
  39the kernel that destroys the owner task (e.g. due to a SEGFAULT), but
  40the kernel cannot help with the cleanup: if there is no 'futex queue'
  41(and in most cases there is none, futexes being fast lightweight locks)
  42then the kernel has no information to clean up after the held lock!
  43Userspace has no chance to clean up after the lock either - userspace is
  44the one that crashes, so it has no opportunity to clean up. Catch-22.
  45
  46In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot
  47is needed to release that futex based lock. This is one of the leading
  48bugreports against yum.
  49
  50To solve this problem, the traditional approach was to extend the vma
  51(virtual memory area descriptor) concept to have a notion of 'pending
  52robust futexes attached to this area'. This approach requires 3 new
  53syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
  54FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
  55they have a robust_head set. This approach has two fundamental problems
  56left:
  57
  58 - it has quite complex locking and race scenarios. The vma-based
  59   approach had been pending for years, but they are still not completely
  60   reliable.
  61
  62 - they have to scan _every_ vma at sys_exit() time, per thread!
  63
  64The second disadvantage is a real killer: pthread_exit() takes around 1
  65microsecond on Linux, but with thousands (or tens of thousands) of vmas
  66every pthread_exit() takes a millisecond or more, also totally
  67destroying the CPU's L1 and L2 caches!
  68
  69This is very much noticeable even for normal process sys_exit_group()
  70calls: the kernel has to do the vma scanning unconditionally! (this is
  71because the kernel has no knowledge about how many robust futexes there
  72are to be cleaned up, because a robust futex might have been registered
  73in another task, and the futex variable might have been simply mmap()-ed
  74into this process's address space).
  75
  76This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that
  77normal kernels can turn it off, but worse than that: the overhead makes
  78robust futexes impractical for any type of generic Linux distribution.
  79
  80So something had to be done.
  81
  82New approach to robust futexes
  83------------------------------
  84
  85At the heart of this new approach there is a per-thread private list of
  86robust locks that userspace is holding (maintained by glibc) - which
  87userspace list is registered with the kernel via a new syscall [this
  88registration happens at most once per thread lifetime]. At do_exit()
  89time, the kernel checks this user-space list: are there any robust futex
  90locks to be cleaned up?
  91
  92In the common case, at do_exit() time, there is no list registered, so
  93the cost of robust futexes is just a simple current->robust_list != NULL
  94comparison. If the thread has registered a list, then normally the list
  95is empty. If the thread/process crashed or terminated in some incorrect
  96way then the list might be non-empty: in this case the kernel carefully
  97walks the list [not trusting it], and marks all locks that are owned by
  98this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if
  99any).
 100
 101The list is guaranteed to be private and per-thread at do_exit() time,
 102so it can be accessed by the kernel in a lockless way.
 103
 104There is one race possible though: since adding to and removing from the
 105list is done after the futex is acquired by glibc, there is a few
 106instructions window for the thread (or process) to die there, leaving
 107the futex hung. To protect against this possibility, userspace (glibc)
 108also maintains a simple per-thread 'list_op_pending' field, to allow the
 109kernel to clean up if the thread dies after acquiring the lock, but just
 110before it could have added itself to the list. Glibc sets this
 111list_op_pending field before it tries to acquire the futex, and clears
 112it after the list-add (or list-remove) has finished.
 113
 114That's all that is needed - all the rest of robust-futex cleanup is done
 115in userspace [just like with the previous patches].
 116
 117Ulrich Drepper has implemented the necessary glibc support for this new
 118mechanism, which fully enables robust mutexes.
 119
 120Key differences of this userspace-list based approach, compared to the
 121vma based method:
 122
 123 - it's much, much faster: at thread exit time, there's no need to loop
 124   over every vma (!), which the VM-based method has to do. Only a very
 125   simple 'is the list empty' op is done.
 126
 127 - no VM changes are needed - 'struct address_space' is left alone.
 128
 129 - no registration of individual locks is needed: robust mutexes dont
 130   need any extra per-lock syscalls. Robust mutexes thus become a very
 131   lightweight primitive - so they dont force the application designer
 132   to do a hard choice between performance and robustness - robust
 133   mutexes are just as fast.
 134
 135 - no per-lock kernel allocation happens.
 136
 137 - no resource limits are needed.
 138
 139 - no kernel-space recovery call (FUTEX_RECOVER) is needed.
 140
 141 - the implementation and the locking is "obvious", and there are no
 142   interactions with the VM.
 143
 144Performance
 145-----------
 146
 147I have benchmarked the time needed for the kernel to process a list of 1
 148million (!) held locks, using the new method [on a 2GHz CPU]:
 149
 150 - with FUTEX_WAIT set [contended mutex]: 130 msecs
 151 - without FUTEX_WAIT set [uncontended mutex]: 30 msecs
 152
 153I have also measured an approach where glibc does the lock notification
 154[which it currently does for !pshared robust mutexes], and that took 256
 155msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls
 156userspace had to do.
 157
 158(1 million held locks are unheard of - we expect at most a handful of
 159locks to be held at a time. Nevertheless it's nice to know that this
 160approach scales nicely.)
 161
 162Implementation details
 163----------------------
 164
 165The patch adds two new syscalls: one to register the userspace list, and
 166one to query the registered list pointer:
 167
 168 asmlinkage long
 169 sys_set_robust_list(struct robust_list_head __user *head,
 170                     size_t len);
 171
 172 asmlinkage long
 173 sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
 174                     size_t __user *len_ptr);
 175
 176List registration is very fast: the pointer is simply stored in
 177current->robust_list. [Note that in the future, if robust futexes become
 178widespread, we could extend sys_clone() to register a robust-list head
 179for new threads, without the need of another syscall.]
 180
 181So there is virtually zero overhead for tasks not using robust futexes,
 182and even for robust futex users, there is only one extra syscall per
 183thread lifetime, and the cleanup operation, if it happens, is fast and
 184straightforward. The kernel doesn't have any internal distinction between
 185robust and normal futexes.
 186
 187If a futex is found to be held at exit time, the kernel sets the
 188following bit of the futex word:
 189
 190        #define FUTEX_OWNER_DIED        0x40000000
 191
 192and wakes up the next futex waiter (if any). User-space does the rest of
 193the cleanup.
 194
 195Otherwise, robust futexes are acquired by glibc by putting the TID into
 196the futex field atomically. Waiters set the FUTEX_WAITERS bit:
 197
 198        #define FUTEX_WAITERS           0x80000000
 199
 200and the remaining bits are for the TID.
 201
 202Testing, architecture support
 203-----------------------------
 204
 205i've tested the new syscalls on x86 and x86_64, and have made sure the
 206parsing of the userspace list is robust [ ;-) ] even if the list is
 207deliberately corrupted.
 208
 209i386 and x86_64 syscalls are wired up at the moment, and Ulrich has
 210tested the new glibc code (on x86_64 and i386), and it works for his
 211robust-mutex testcases.
 212
 213All other architectures should build just fine too - but they wont have
 214the new syscalls yet.
 215
 216Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
 217inline function before writing up the syscalls (that function returns
 218-ENOSYS right now).
 219
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.