nptl/sem_wait.c

   1 /* sem_wait -- wait on a semaphore.  Generic futex-using version.
   2    Copyright (C) 2003-2017 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    <http://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, NULL);
  43 }
  44 versioned_symbol (libpthread, __new_sem_wait, sem_wait, GLIBC_2_1);
  45
  46 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  47 int
  48 attribute_compat_text_section
  49 __old_sem_wait (sem_t *sem)
  50 {
  51   int *futex = (int *) sem;
  52   int err;
  53
  54   do
  55     {
  56       if (atomic_decrement_if_positive (futex) > 0)
  57         return 0;
  58
  59       /* Enable asynchronous cancellation.  Required by the standard.  */
  60       int oldtype = __pthread_enable_asynccancel ();
  61
  62       /* Always assume the semaphore is shared.  */
  63       err = lll_futex_wait (futex, 0, LLL_SHARED);
  64
  65       /* Disable asynchronous cancellation.  */
  66       __pthread_disable_asynccancel (oldtype);
  67     }
  68   while (err == 0 || err == -EWOULDBLOCK);
  69
  70   __set_errno (-err);
  71   return -1;
  72 }
  73
  74 compat_symbol (libpthread, __old_sem_wait, sem_wait, GLIBC_2_0);
  75 #endif
  76
  77 int
  78 __new_sem_trywait (sem_t *sem)
  79 {
  80   /* We must not fail spuriously, so require a definitive result even if this
  81      may lead to a long execution time.  */
  82   if (__new_sem_wait_fast ((struct new_sem *) sem, 1) == 0)
  83     return 0;
  84   __set_errno (EAGAIN);
  85   return -1;
  86 }
  87 versioned_symbol (libpthread, __new_sem_trywait, sem_trywait, GLIBC_2_1);
  88 #if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
  89 int
  90 __old_sem_trywait (sem_t *sem)
  91 {
  92   int *futex = (int *) sem;
  93   int val;
  94
  95   if (*futex > 0)
  96     {
  97       val = atomic_decrement_if_positive (futex);
  98       if (val > 0)
  99         return 0;
 100     }
 101
 102   __set_errno (EAGAIN);
 103   return -1;
 104 }
 105 compat_symbol (libpthread, __old_sem_trywait, sem_trywait, GLIBC_2_0);
 106 #endif