[PATCH 06/12] rwsem: wake queued readers when other readers are active
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From: Michel Lespinasse on 11 May 2010 23:30 This change addresses the following situation: - Thread A holds the rwsem for write - Thread B tries to acquire the rwsem for read; blocks & gets queued - Thread A releases the rwsem, notices active count goes back to 0 - Thread C acquires the rwsem for read - Thread A acquires the spinlock & tries to wake thread B, but fails because active count is not zero anymore. In this situation, it would be perfectly fine to let threads B and C work in parallel as they each only want a read acquire on the rwsem. We can recognize this situation and let A wake B as long as there are no active writers on the rwsem. Signed-off-by: Michel Lespinasse <walken(a)google.com> --- lib/rwsem.c | 6 +++--- 1 files changed, 3 insertions(+), 3 deletions(-) diff --git a/lib/rwsem.c b/lib/rwsem.c index 92c8f8e..9d0899b 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -115,8 +115,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) retry_readers: oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; - if (!downgrading && (oldcount & RWSEM_ACTIVE_MASK)) - /* Someone grabbed the sem already */ + if (!downgrading && (oldcount < RWSEM_WAITING_BIAS)) + /* Someone grabbed the sem for write already */ goto undo_readers; next = sem->wait_list.next; @@ -143,7 +143,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) goto retry_writer; undo_readers: - if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) + if (rwsem_atomic_update(-adjustment, sem) < RWSEM_WAITING_BIAS) goto out; goto retry_readers; } -- 1.7.0.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |