nptl/sem_wait.c

   1 /* sem_wait -- wait on a semaphore.  Generic futex-using version.
   2    Copyright (C) 2003-2020 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 "sem_waitcommon.c"
  22
  23 int
  24 __new_sem_wait (sem_t *sem)
  25 {
  26   /* We need to check whether we need to act upon a cancellation request here
  27      because POSIX specifies that cancellation points "shall occur" in
  28      sem_wait and sem_timedwait, which also means that they need to check
  29      this regardless whether they block or not (unlike "may occur"
  30      functions).  See the POSIX Rationale for this requirement: Section
  31      "Thread Cancellation Overview" [1] and austin group issue #1076 [2]
  32      for thoughs on why this may be a suboptimal design.
  33
  34      [1] http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xsh_chap02.html
  35      [2] http://austingroupbugs.net/view.php?id=1076 for thoughts on why this
  36    */
  37   __pthread_testcancel ();
  38
  39   if (__new_sem_wait_fast ((struct new_sem *) sem, 0) == 0)
  40     return 0;
  41   else
  42     return __new_sem_wait_slow ((struct new_sem *) sem,
  43                                 CLOCK_REALTIME, NULL);
  44 }
  45 versioned_symbol (libpthread, __new_sem_wait, sem_wait, GLIBC_2_1);
  46
  47 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  48 int
  49 attribute_compat_text_section
  50 __old_sem_wait (sem_t *sem)
  51 {
  52   int *futex = (int *) sem;
  53   int err;
  54
  55   do
  56     {
  57       if (atomic_decrement_if_positive (futex) > 0)
  58         return 0;
  59
  60       /* Always assume the semaphore is shared.  */
  61       err = lll_futex_wait_cancel (futex, 0, LLL_SHARED);
  62     }
  63   while (err == 0 || err == -EWOULDBLOCK);
  64
  65   __set_errno (-err);
  66   return -1;
  67 }
  68
  69 compat_symbol (libpthread, __old_sem_wait, sem_wait, GLIBC_2_0);
  70 #endif
  71
  72 int
  73 __new_sem_trywait (sem_t *sem)
  74 {
  75   /* We must not fail spuriously, so require a definitive result even if this
  76      may lead to a long execution time.  */
  77   if (__new_sem_wait_fast ((struct new_sem *) sem, 1) == 0)
  78     return 0;
  79   __set_errno (EAGAIN);
  80   return -1;
  81 }
  82 versioned_symbol (libpthread, __new_sem_trywait, sem_trywait, GLIBC_2_1);
  83 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  84 int
  85 __old_sem_trywait (sem_t *sem)
  86 {
  87   int *futex = (int *) sem;
  88   int val;
  89
  90   if (*futex > 0)
  91     {
  92       val = atomic_decrement_if_positive (futex);
  93       if (val > 0)
  94         return 0;
  95     }
  96
  97   __set_errno (EAGAIN);
  98   return -1;
  99 }
 100 compat_symbol (libpthread, __old_sem_trywait, sem_trywait, GLIBC_2_0);
 101 #endif