sysdeps/nptl/lowlevellock.h

   1 /* Low-level lock implementation.  Generic futex-based version.
   2    Copyright (C) 2005-2019 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library.  If not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _LOWLEVELLOCK_H
  20 #define _LOWLEVELLOCK_H 1
  21
  22 #include <atomic.h>
  23 #include <lowlevellock-futex.h>
  24
  25 /* Low-level locks use a combination of atomic operations (to acquire and
  26    release lock ownership) and futex operations (to block until the state
  27    of a lock changes).  A lock can be in one of three states:
  28    0:  not acquired,
  29    1:  acquired with no waiters; no other threads are blocked or about to block
  30        for changes to the lock state,
  31    >1: acquired, possibly with waiters; there may be other threads blocked or
  32        about to block for changes to the lock state.
  33
  34    We expect that the common case is an uncontended lock, so we just need
  35    to transition the lock between states 0 and 1; releasing the lock does
  36    not need to wake any other blocked threads.  If the lock is contended
  37    and a thread decides to block using a futex operation, then this thread
  38    needs to first change the state to >1; if this state is observed during
  39    lock release, the releasing thread will wake one of the potentially
  40    blocked threads.
  41
  42    Much of this code takes a 'private' parameter.  This may be:
  43    LLL_PRIVATE: lock only shared within a process
  44    LLL_SHARED:  lock may be shared across processes.
  45
  46    Condition variables contain an optimization for broadcasts that requeues
  47    waiting threads on a lock's futex.  Therefore, there is a special
  48    variant of the locks (whose name contains "cond") that makes sure to
  49    always set the lock state to >1 and not just 1.
  50
  51    Robust locks set the lock to the id of the owner.  This allows detection
  52    of the case where the owner exits without releasing the lock.  Flags are
  53    OR'd with the owner id to record additional information about lock state.
  54    Therefore the states of robust locks are:
  55     0: not acquired
  56    id: acquired (by user identified by id & FUTEX_TID_MASK)
  57
  58    The following flags may be set in the robust lock value:
  59    FUTEX_WAITERS     - possibly has waiters
  60    FUTEX_OWNER_DIED  - owning user has exited without releasing the futex.  */
  61
  62
  63 /* If LOCK is 0 (not acquired), set to 1 (acquired with no waiters) and return
  64    0.  Otherwise leave lock unchanged and return non-zero to indicate that the
  65    lock was not acquired.  */
  66 #define lll_trylock(lock)       \
  67   __glibc_unlikely (atomic_compare_and_exchange_bool_acq (&(lock), 1, 0))
  68
  69 /* If LOCK is 0 (not acquired), set to 2 (acquired, possibly with waiters) and
  70    return 0.  Otherwise leave lock unchanged and return non-zero to indicate
  71    that the lock was not acquired.  */
  72 #define lll_cond_trylock(lock)  \
  73   __glibc_unlikely (atomic_compare_and_exchange_bool_acq (&(lock), 2, 0))
  74
  75 extern void __lll_lock_wait_private (int *futex) attribute_hidden;
  76 extern void __lll_lock_wait (int *futex, int private) attribute_hidden;
  77
  78 /* This is an expression rather than a statement even though its value is
  79    void, so that it can be used in a comma expression or as an expression
  80    that's cast to void.  */
  81 /* The inner conditional compiles to a call to __lll_lock_wait_private if
  82    private is known at compile time to be LLL_PRIVATE, and to a call to
  83    __lll_lock_wait otherwise.  */
  84 /* If FUTEX is 0 (not acquired), set to 1 (acquired with no waiters) and
  85    return.  Otherwise, ensure that it is >1 (acquired, possibly with waiters)
  86    and then block until we acquire the lock, at which point FUTEX will still be
  87    >1.  The lock is always acquired on return.  */
  88 #define __lll_lock(futex, private)                                      \
  89   ((void)                                                               \
  90    ({                                                                   \
  91      int *__futex = (futex);                                            \
  92      if (__glibc_unlikely                                               \
  93          (atomic_compare_and_exchange_bool_acq (__futex, 1, 0)))        \
  94        {                                                                \
  95          if (__builtin_constant_p (private) && (private) == LLL_PRIVATE) \
  96            __lll_lock_wait_private (__futex);                           \
  97          else                                                           \
  98            __lll_lock_wait (__futex, private);                          \
  99        }                                                                \
 100    }))
 101 #define lll_lock(futex, private)        \
 102   __lll_lock (&(futex), private)
 103
 104
 105 /* This is an expression rather than a statement even though its value is
 106    void, so that it can be used in a comma expression or as an expression
 107    that's cast to void.  */
 108 /* Unconditionally set FUTEX to 2 (acquired, possibly with waiters).  If FUTEX
 109    was 0 (not acquired) then return.  Otherwise, block until the lock is
 110    acquired, at which point FUTEX is 2 (acquired, possibly with waiters).  The
 111    lock is always acquired on return.  */
 112 #define __lll_cond_lock(futex, private)                                 \
 113   ((void)                                                               \
 114    ({                                                                   \
 115      int *__futex = (futex);                                            \
 116      if (__glibc_unlikely (atomic_exchange_acq (__futex, 2) != 0))      \
 117        __lll_lock_wait (__futex, private);                              \
 118    }))
 119 #define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)
 120
 121
 122 extern int __lll_timedlock_wait (int *futex, const struct timespec *,
 123                                  int private) attribute_hidden;
 124
 125
 126 /* As __lll_lock, but with a timeout.  If the timeout occurs then return
 127    ETIMEDOUT.  If ABSTIME is invalid, return EINVAL.  */
 128 #define __lll_timedlock(futex, abstime, private)                \
 129   ({                                                            \
 130     int *__futex = (futex);                                     \
 131     int __val = 0;                                              \
 132                                                                 \
 133     if (__glibc_unlikely                                        \
 134         (atomic_compare_and_exchange_bool_acq (__futex, 1, 0))) \
 135       __val = __lll_timedlock_wait (__futex, abstime, private); \
 136     __val;                                                      \
 137   })
 138 #define lll_timedlock(futex, abstime, private)  \
 139   __lll_timedlock (&(futex), abstime, private)
 140
 141
 142 /* This is an expression rather than a statement even though its value is
 143    void, so that it can be used in a comma expression or as an expression
 144    that's cast to void.  */
 145 /* Unconditionally set FUTEX to 0 (not acquired), releasing the lock.  If FUTEX
 146    was >1 (acquired, possibly with waiters), then wake any waiters.  The waiter
 147    that acquires the lock will set FUTEX to >1.
 148    Evaluate PRIVATE before releasing the lock so that we do not violate the
 149    mutex destruction requirements.  Specifically, we need to ensure that
 150    another thread can destroy the mutex (and reuse its memory) once it
 151    acquires the lock and when there will be no further lock acquisitions;
 152    thus, we must not access the lock after releasing it, or those accesses
 153    could be concurrent with mutex destruction or reuse of the memory.  */
 154 #define __lll_unlock(futex, private)                    \
 155   ((void)                                               \
 156    ({                                                   \
 157      int *__futex = (futex);                            \
 158      int __private = (private);                         \
 159      int __oldval = atomic_exchange_rel (__futex, 0);   \
 160      if (__glibc_unlikely (__oldval > 1))               \
 161        lll_futex_wake (__futex, 1, __private);          \
 162    }))
 163 #define lll_unlock(futex, private)      \
 164   __lll_unlock (&(futex), private)
 165
 166
 167 #define lll_islocked(futex) \
 168   ((futex) != LLL_LOCK_INITIALIZER)
 169
 170
 171 /* Our internal lock implementation is identical to the binary-compatible
 172    mutex implementation. */
 173
 174 /* Initializers for lock.  */
 175 #define LLL_LOCK_INITIALIZER            (0)
 176 #define LLL_LOCK_INITIALIZER_LOCKED     (1)
 177
 178 #endif  /* lowlevellock.h */