nptl/sem_post.c

   1 /* sem_post -- post to a POSIX semaphore.  Generic futex-using version.
   2    Copyright (C) 2003-2015 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Jakub Jelinek <jakub@redhat.com>, 2003.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <http://www.gnu.org/licenses/>.  */
  19
  20 #include <atomic.h>
  21 #include <errno.h>
  22 #include <sysdep.h>
  23 #include <lowlevellock.h>
  24 #include <internaltypes.h>
  25 #include <semaphore.h>
  26
  27 #include <shlib-compat.h>
  28
  29 /* Wrapper for lll_futex_wake, with error checking.
  30    TODO Remove when cleaning up the futex API throughout glibc.  */
  31 static __always_inline void
  32 futex_wake (unsigned int* futex, int processes_to_wake, int private)
  33 {
  34   int res = lll_futex_wake (futex, processes_to_wake, private);
  35   /* No error.  Ignore the number of woken processes.  */
  36   if (res >= 0)
  37     return;
  38   switch (res)
  39     {
  40     case -EFAULT: /* Could have happened due to memory reuse.  */
  41     case -EINVAL: /* Could be either due to incorrect alignment (a bug in
  42                      glibc or in the application) or due to memory being
  43                      reused for a PI futex.  We cannot distinguish between the
  44                      two causes, and one of them is correct use, so we do not
  45                      act in this case.  */
  46       return;
  47     case -ENOSYS: /* Must have been caused by a glibc bug.  */
  48     /* No other errors are documented at this time.  */
  49     default:
  50       abort ();
  51     }
  52 }
  53
  54
  55 /* See sem_wait for an explanation of the algorithm.  */
  56 int
  57 __new_sem_post (sem_t *sem)
  58 {
  59   struct new_sem *isem = (struct new_sem *) sem;
  60   int private = isem->private;
  61
  62 #if __HAVE_64B_ATOMICS
  63   /* Add a token to the semaphore.  We use release MO to make sure that a
  64      thread acquiring this token synchronizes with us and other threads that
  65      added tokens before (the release sequence includes atomic RMW operations
  66      by other threads).  */
  67   /* TODO Use atomic_fetch_add to make it scale better than a CAS loop?  */
  68   uint64_t d = atomic_load_relaxed (&isem->data);
  69   do
  70     {
  71       if ((d & SEM_VALUE_MASK) == SEM_VALUE_MAX)
  72         {
  73           __set_errno (EOVERFLOW);
  74           return -1;
  75         }
  76     }
  77   while (!atomic_compare_exchange_weak_release (&isem->data, &d, d + 1));
  78
  79   /* If there is any potentially blocked waiter, wake one of them.  */
  80   if ((d >> SEM_NWAITERS_SHIFT) > 0)
  81     futex_wake (((unsigned int *) &isem->data) + SEM_VALUE_OFFSET, 1, private);
  82 #else
  83   /* Add a token to the semaphore.  Similar to 64b version.  */
  84   unsigned int v = atomic_load_relaxed (&isem->value);
  85   do
  86     {
  87       if ((v >> SEM_VALUE_SHIFT) == SEM_VALUE_MAX)
  88         {
  89           __set_errno (EOVERFLOW);
  90           return -1;
  91         }
  92     }
  93   while (!atomic_compare_exchange_weak_release
  94          (&isem->value, &v, v + (1 << SEM_VALUE_SHIFT)));
  95
  96   /* If there is any potentially blocked waiter, wake one of them.  */
  97   if ((v & SEM_NWAITERS_MASK) != 0)
  98     futex_wake (&isem->value, 1, private);
  99 #endif
 100
 101   return 0;
 102 }
 103 versioned_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_1);
 104
 105
 106 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
 107 int
 108 attribute_compat_text_section
 109 __old_sem_post (sem_t *sem)
 110 {
 111   int *futex = (int *) sem;
 112
 113   /* We must need to synchronize with consumers of this token, so the atomic
 114      increment must have release MO semantics.  */
 115   atomic_write_barrier ();
 116   (void) atomic_increment_val (futex);
 117   /* We always have to assume it is a shared semaphore.  */
 118   int err = lll_futex_wake (futex, 1, LLL_SHARED);
 119   if (__builtin_expect (err, 0) < 0)
 120     {
 121       __set_errno (-err);
 122       return -1;
 123     }
 124   return 0;
 125 }
 126 compat_symbol (libpthread, __old_sem_post, sem_post, GLIBC_2_0);
 127 #endif