| /* Copyright (C) 2002-2018 Free Software Foundation, Inc. |
| This file is part of the GNU C Library. |
| Contributed by Ulrich Drepper <drepper@redhat.com>, 2002. |
| |
| The GNU C Library is free software; you can redistribute it and/or |
| modify it under the terms of the GNU Lesser General Public |
| License as published by the Free Software Foundation; either |
| version 2.1 of the License, or (at your option) any later version. |
| |
| The GNU C Library is distributed in the hope that it will be useful, |
| but WITHOUT ANY WARRANTY; without even the implied warranty of |
| MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| Lesser General Public License for more details. |
| |
| You should have received a copy of the GNU Lesser General Public |
| License along with the GNU C Library; if not, see |
| <http://www.gnu.org/licenses/>. */ |
| |
| #include <errno.h> |
| #include "pthreadP.h" |
| #include <atomic.h> |
| |
| /* See pthread_rwlock_common.c for an overview. */ |
| int |
| __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock) |
| { |
| /* When in a trywrlock, we can acquire the write lock if it is in states |
| #1 (idle and read phase) and #5 (idle and write phase), and also in #6 |
| (readers waiting, write phase) if we prefer writers. |
| If we observe any other state, we are allowed to fail and do not need to |
| "synchronize memory" as specified by POSIX (hence relaxed MO is |
| sufficient for the first load and the CAS failure path). |
| We face a similar issue as in tryrdlock in that we need to both avoid |
| live-locks / starvation and must not fail spuriously (see there for |
| further comments) -- and thus must loop until we get a definitive |
| observation or state change. */ |
| unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers); |
| bool prefer_writer = |
| (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP); |
| while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0) |
| && (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0) |
| || (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0)))) |
| { |
| /* Try to transition to states #7 or #8 (i.e., acquire the lock). */ |
| if (atomic_compare_exchange_weak_acquire ( |
| &rwlock->__data.__readers, &r, |
| r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED)) |
| { |
| atomic_store_relaxed (&rwlock->__data.__writers_futex, 1); |
| atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1); |
| atomic_store_relaxed (&rwlock->__data.__cur_writer, |
| THREAD_GETMEM (THREAD_SELF, tid)); |
| return 0; |
| } |
| /* TODO Back-off. */ |
| /* See above. */ |
| } |
| return EBUSY; |
| } |
| |
| strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock) |
