nptl/sem_wait.c

   1 /* sem_wait -- wait on a 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 Paul Mackerras <paulus@au.ibm.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 <lowlevellock.h>       /* lll_futex* used by the old code.  */
  21 #include "semaphoreP.h"
  22 #include "sem_waitcommon.c"
  23
  24 int
  25 __new_sem_wait (sem_t *sem)
  26 {
  27   /* We need to check whether we need to act upon a cancellation request here
  28      because POSIX specifies that cancellation points "shall occur" in
  29      sem_wait and sem_timedwait, which also means that they need to check
  30      this regardless whether they block or not (unlike "may occur"
  31      functions).  See the POSIX Rationale for this requirement: Section
  32      "Thread Cancellation Overview" [1] and austin group issue #1076 [2]
  33      for thoughs on why this may be a suboptimal design.
  34
  35      [1] http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xsh_chap02.html
  36      [2] http://austingroupbugs.net/view.php?id=1076 for thoughts on why this
  37    */
  38   __pthread_testcancel ();
  39
  40   if (__new_sem_wait_fast ((struct new_sem *) sem, 0) == 0)
  41     return 0;
  42   else
  43     return __new_sem_wait_slow64 ((struct new_sem *) sem,
  44                                   CLOCK_REALTIME, NULL);
  45 }
  46 versioned_symbol (libpthread, __new_sem_wait, sem_wait, GLIBC_2_1);
  47
  48 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  49 int
  50 attribute_compat_text_section
  51 __old_sem_wait (sem_t *sem)
  52 {
  53   int *futex = (int *) sem;
  54   int err;
  55
  56   do
  57     {
  58       if (atomic_decrement_if_positive (futex) > 0)
  59         return 0;
  60
  61       /* Always assume the semaphore is shared.  */
  62       err = lll_futex_wait_cancel (futex, 0, LLL_SHARED);
  63     }
  64   while (err == 0 || err == -EWOULDBLOCK);
  65
  66   __set_errno (-err);
  67   return -1;
  68 }
  69
  70 compat_symbol (libpthread, __old_sem_wait, sem_wait, GLIBC_2_0);
  71 #endif
  72
  73 int
  74 __new_sem_trywait (sem_t *sem)
  75 {
  76   /* We must not fail spuriously, so require a definitive result even if this
  77      may lead to a long execution time.  */
  78   if (__new_sem_wait_fast ((struct new_sem *) sem, 1) == 0)
  79     return 0;
  80   __set_errno (EAGAIN);
  81   return -1;
  82 }
  83 versioned_symbol (libpthread, __new_sem_trywait, sem_trywait, GLIBC_2_1);
  84 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  85 int
  86 __old_sem_trywait (sem_t *sem)
  87 {
  88   int *futex = (int *) sem;
  89   int val;
  90
  91   if (*futex > 0)
  92     {
  93       val = atomic_decrement_if_positive (futex);
  94       if (val > 0)
  95         return 0;
  96     }
  97
  98   __set_errno (EAGAIN);
  99   return -1;
 100 }
 101 compat_symbol (libpthread, __old_sem_trywait, sem_trywait, GLIBC_2_0);
 102 #endif