nptl/pthread_rwlock_trywrlock.c

   1 /* Copyright (C) 2002-2020 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include "pthreadP.h"
  21 #include <atomic.h>
  22
  23 /* See pthread_rwlock_common.c for an overview.  */
  24 int
  25 __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
  26 {
  27   /* When in a trywrlock, we can acquire the write lock if it is in states
  28      #1 (idle and read phase) and #5 (idle and write phase), and also in #6
  29      (readers waiting, write phase) if we prefer writers.
  30      If we observe any other state, we are allowed to fail and do not need to
  31      "synchronize memory" as specified by POSIX (hence relaxed MO is
  32      sufficient for the first load and the CAS failure path).
  33      We face a similar issue as in tryrdlock in that we need to both avoid
  34      live-locks / starvation and must not fail spuriously (see there for
  35      further comments) -- and thus must loop until we get a definitive
  36      observation or state change.  */
  37   unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
  38   bool prefer_writer =
  39       (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
  40   while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
  41       && (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
  42           || (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
  43     {
  44       /* Try to transition to states #7 or #8 (i.e., acquire the lock).  */
  45       if (atomic_compare_exchange_weak_acquire (
  46           &rwlock->__data.__readers, &r,
  47           r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
  48         {
  49           /* We have become the primary writer and we cannot have shared
  50              the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we
  51              can simply enable blocking (see full wrlock code).  */
  52           atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
  53           /* If we started a write phase, we need to enable readers to
  54              wait.  If we did not, we must not change it because other threads
  55              may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime.  */
  56           if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
  57             atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
  58           atomic_store_relaxed (&rwlock->__data.__cur_writer,
  59               THREAD_GETMEM (THREAD_SELF, tid));
  60           return 0;
  61         }
  62       /* TODO Back-off.  */
  63       /* See above.  */
  64     }
  65   return EBUSY;
  66 }
  67
  68 strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock)