nptl/sem_post.c

   1 /* sem_post -- post to a POSIX semaphore.  Generic futex-using version.
   2    Copyright (C) 2003-2021 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    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <atomic.h>
  20 #include <errno.h>
  21 #include <sysdep.h>
  22 #include <lowlevellock.h>       /* lll_futex* used by the old code.  */
  23 #include <futex-internal.h>
  24 #include <internaltypes.h>
  25 #include <semaphore.h>
  26
  27 #include <shlib-compat.h>
  28
  29
  30 /* See sem_wait for an explanation of the algorithm.  */
  31 int
  32 __new_sem_post (sem_t *sem)
  33 {
  34   struct new_sem *isem = (struct new_sem *) sem;
  35   int private = isem->private;
  36
  37 #if __HAVE_64B_ATOMICS
  38   /* Add a token to the semaphore.  We use release MO to make sure that a
  39      thread acquiring this token synchronizes with us and other threads that
  40      added tokens before (the release sequence includes atomic RMW operations
  41      by other threads).  */
  42   /* TODO Use atomic_fetch_add to make it scale better than a CAS loop?  */
  43   uint64_t d = atomic_load_relaxed (&isem->data);
  44   do
  45     {
  46       if ((d & SEM_VALUE_MASK) == SEM_VALUE_MAX)
  47         {
  48           __set_errno (EOVERFLOW);
  49           return -1;
  50         }
  51     }
  52   while (!atomic_compare_exchange_weak_release (&isem->data, &d, d + 1));
  53
  54   /* If there is any potentially blocked waiter, wake one of them.  */
  55   if ((d >> SEM_NWAITERS_SHIFT) > 0)
  56     futex_wake (((unsigned int *) &isem->data) + SEM_VALUE_OFFSET, 1, private);
  57 #else
  58   /* Add a token to the semaphore.  Similar to 64b version.  */
  59   unsigned int v = atomic_load_relaxed (&isem->value);
  60   do
  61     {
  62       if ((v >> SEM_VALUE_SHIFT) == SEM_VALUE_MAX)
  63         {
  64           __set_errno (EOVERFLOW);
  65           return -1;
  66         }
  67     }
  68   while (!atomic_compare_exchange_weak_release
  69          (&isem->value, &v, v + (1 << SEM_VALUE_SHIFT)));
  70
  71   /* If there is any potentially blocked waiter, wake one of them.  */
  72   if ((v & SEM_NWAITERS_MASK) != 0)
  73     futex_wake (&isem->value, 1, private);
  74 #endif
  75
  76   return 0;
  77 }
  78 versioned_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_34);
  79
  80 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
  81 compat_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_1);
  82 #endif
  83
  84 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  85 int
  86 attribute_compat_text_section
  87 __old_sem_post (sem_t *sem)
  88 {
  89   unsigned int *futex = (unsigned int *) sem;
  90
  91   /* We must need to synchronize with consumers of this token, so the atomic
  92      increment must have release MO semantics.  */
  93   atomic_write_barrier ();
  94   (void) atomic_increment_val (futex);
  95   /* We always have to assume it is a shared semaphore.  */
  96   futex_wake (futex, 1, LLL_SHARED);
  97   return 0;
  98 }
  99 compat_symbol (libpthread, __old_sem_post, sem_post, GLIBC_2_0);
 100 #endif