nptl/pthread_cond_destroy.c

   1 /* Copyright (C) 2002-2022 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <shlib-compat.h>
  20 #include "pthreadP.h"
  21 #include <stap-probe.h>
  22 #include <atomic.h>
  23 #include <futex-internal.h>
  24
  25 #include "pthread_cond_common.c"
  26
  27 /* See __pthread_cond_wait for a high-level description of the algorithm.
  28
  29    A correct program must make sure that no waiters are blocked on the condvar
  30    when it is destroyed, and that there are no concurrent signals or
  31    broadcasts.  To wake waiters reliably, the program must signal or
  32    broadcast while holding the mutex or after having held the mutex.  It must
  33    also ensure that no signal or broadcast are still pending to unblock
  34    waiters; IOW, because waiters can wake up spuriously, the program must
  35    effectively ensure that destruction happens after the execution of those
  36    signal or broadcast calls.
  37    Thus, we can assume that all waiters that are still accessing the condvar
  38    have been woken.  We wait until they have confirmed to have woken up by
  39    decrementing __wrefs.  */
  40 int
  41 __pthread_cond_destroy (pthread_cond_t *cond)
  42 {
  43   LIBC_PROBE (cond_destroy, 1, cond);
  44
  45   /* Set the wake request flag.  We could also spin, but destruction that is
  46      concurrent with still-active waiters is probably neither common nor
  47      performance critical.  Acquire MO to synchronize with waiters confirming
  48      that they finished.  */
  49   unsigned int wrefs = atomic_fetch_or_acquire (&cond->__data.__wrefs, 4);
  50   int private = __condvar_get_private (wrefs);
  51   while (wrefs >> 3 != 0)
  52     {
  53       futex_wait_simple (&cond->__data.__wrefs, wrefs, private);
  54       /* See above.  */
  55       wrefs = atomic_load_acquire (&cond->__data.__wrefs);
  56     }
  57   /* The memory the condvar occupies can now be reused.  */
  58   return 0;
  59 }
  60 libc_hidden_def (__pthread_cond_destroy)
  61 versioned_symbol (libc, __pthread_cond_destroy,
  62                   pthread_cond_destroy, GLIBC_2_3_2);