[PATCH 06/11] rwsem: wake queued readers when writer blocks on active read lock
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From: Michel Lespinasse on 24 May 2010 16:40 This change addresses the following situation: - Thread A acquires the rwsem for read - Thread B tries to acquire the rwsem for write, notices there is already an active owner for the rwsem. - Thread C tries to acquire the rwsem for read, notices that thread B already tried to acquire it. - Thread C grabs the spinlock and queues itself on the wait queue. - Thread B grabs the spinlock and queues itself behind C. At this point A is the only remaining active owner on the rwsem. In this situation thread B could notice that it was the last active writer on the rwsem, and decide to wake C to let it proceed in parallel with A since they both only want the rwsem for read. Signed-off-by: Michel Lespinasse <walken(a)google.com> --- lib/rwsem.c | 19 +++++++++++++++---- 1 files changed, 15 insertions(+), 4 deletions(-) diff --git a/lib/rwsem.c b/lib/rwsem.c index a3e68bf..318d435 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -67,6 +67,9 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) goto readers_only; if (wake_type == RWSEM_WAKE_READ_OWNED) + /* Another active reader was observed, so wakeup is not + * likely to succeed. Save the atomic op. + */ goto out; /* There's a writer at the front of the queue - try to grant it the @@ -111,8 +114,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) * count adjustment pretty soon. */ if (wake_type == RWSEM_WAKE_ANY && - (rwsem_atomic_update(0, sem) & RWSEM_ACTIVE_MASK)) - /* Someone grabbed the sem already */ + rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) + /* Someone grabbed the sem for write already */ goto out; /* Grant an infinite number of read locks to the readers at the front @@ -187,9 +190,17 @@ rwsem_down_failed_common(struct rw_semaphore *sem, /* we're now waiting on the lock, but no longer actively locking */ count = rwsem_atomic_update(adjustment, sem); - /* if there are no active locks, wake the front queued process(es) up */ - if (!(count & RWSEM_ACTIVE_MASK)) + /* If there are no active locks, wake the front queued process(es) up. + * + * Alternatively, if we're called from a failed down_write(), there + * were already threads queued before us and there are no active + * writers, the lock must be read owned; so we try to wake any read + * locks that were queued ahead of us. */ + if (count == RWSEM_WAITING_BIAS) sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); + else if (count > RWSEM_WAITING_BIAS && + adjustment == -RWSEM_ACTIVE_WRITE_BIAS) + sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); spin_unlock_irq(&sem->wait_lock); -- 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/ |