diff options
Diffstat (limited to 'nptl/pthread_rwlock_trywrlock.c')
| -rw-r--r-- | nptl/pthread_rwlock_trywrlock.c | 53 |
1 files changed, 33 insertions, 20 deletions
diff --git a/nptl/pthread_rwlock_trywrlock.c b/nptl/pthread_rwlock_trywrlock.c index 251ece133f..0d9ccaf79c 100644 --- a/nptl/pthread_rwlock_trywrlock.c +++ b/nptl/pthread_rwlock_trywrlock.c @@ -18,31 +18,44 @@ #include <errno.h> #include "pthreadP.h" -#include <lowlevellock.h> -#include <elide.h> - +#include <atomic.h> +/* See pthread_rwlock_common.c for an overview. */ int __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock) { - int result = EBUSY; - - if (ELIDE_TRYLOCK (rwlock->__data.__rwelision, - rwlock->__data.__lock == 0 - && rwlock->__data.__nr_readers == 0 - && rwlock->__data.__writer, 1)) - return 0; - - lll_lock (rwlock->__data.__lock, rwlock->__data.__shared); - - if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0) + /* 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)))) { - rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid); - result = 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. */ } - - lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared); - - return result; + return EBUSY; } + strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock) |