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