diff options
| -rw-r--r-- | kernel/locking/rwbase_rt.c | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c index 48fbbccdce3f..88191f6e252c 100644 --- a/kernel/locking/rwbase_rt.c +++ b/kernel/locking/rwbase_rt.c @@ -41,6 +41,12 @@ * The risk of writer starvation is there, but the pathological use cases * which trigger it are not necessarily the typical RT workloads. * + * Fast-path orderings: + * The lock/unlock of readers can run in fast paths: lock and unlock are only + * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE + * semantics of rwbase_rt. Atomic ops should thus provide _acquire() + * and _release() (or stronger). + * * Common code shared between RT rw_semaphore and rwlock */ @@ -53,6 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) * set. */ for (r = atomic_read(&rwb->readers); r < 0;) { + /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */ if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1))) return 1; } @@ -162,6 +169,8 @@ static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb, /* * rwb->readers can only hit 0 when a writer is waiting for the * active readers to leave the critical section. + * + * dec_and_test() is fully ordered, provides RELEASE. */ if (unlikely(atomic_dec_and_test(&rwb->readers))) __rwbase_read_unlock(rwb, state); @@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias, { struct rt_mutex_base *rtm = &rwb->rtmutex; - atomic_add(READER_BIAS - bias, &rwb->readers); + /* + * _release() is needed in case that reader is in fast path, pairing + * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE + */ + (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers); raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); rwbase_rtmutex_unlock(rtm); } @@ -201,7 +214,11 @@ static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb) /* Can do without CAS because we're serialized by wait_lock. */ lockdep_assert_held(&rwb->rtmutex.wait_lock); - if (!atomic_read(&rwb->readers)) { + /* + * _acquire is needed in case the reader is in the fast path, pairing + * with rwbase_read_unlock(), provides ACQUIRE. + */ + if (!atomic_read_acquire(&rwb->readers)) { atomic_set(&rwb->readers, WRITER_BIAS); return 1; } |