Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754193Ab3COKyx (ORCPT ); Fri, 15 Mar 2013 06:54:53 -0400 Received: from mail-pb0-f44.google.com ([209.85.160.44]:61598 "EHLO mail-pb0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754168Ab3COKyv (ORCPT ); Fri, 15 Mar 2013 06:54:51 -0400 From: Michel Lespinasse To: Alex Shi , Ingo Molnar , David Howells , Peter Zijlstra , Thomas Gleixner , Yuanhan Liu , Rik van Riel , Peter Hurley , Dave Chinner Cc: Andrew Morton , linux-kernel@vger.kernel.org Subject: [PATCH v2 12/13] rwsem: do not block readers at head of queue if other readers are active Date: Fri, 15 Mar 2013 03:54:28 -0700 Message-Id: <1363344869-15732-13-git-send-email-walken@google.com> X-Mailer: git-send-email 1.8.1.3 In-Reply-To: <1363344869-15732-1-git-send-email-walken@google.com> References: <1363344869-15732-1-git-send-email-walken@google.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1634 Lines: 43 This change fixes a race condition where a reader might determine it needs to block, but by the time it acquires the wait_lock the rwsem has active readers and no queued waiters. In this situation the reader can just in parallel with the existing active readers; it does not need to block until the active readers complete. Thanks to Peter Hurley for noticing this possible race. Signed-off-by: Michel Lespinasse --- lib/rwsem.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/lib/rwsem.c b/lib/rwsem.c index 09bf03e7808c..4e4c8893dc00 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -162,8 +162,14 @@ struct rw_semaphore __sched *rwsem_down_read_failed(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). */ - if (!(count & RWSEM_ACTIVE_MASK)) + /* If there are no active locks, wake the front queued process(es). + * + * If there are no writers and we are first in the queue, + * wake our own waiter to join the existing active readers ! + */ + if (count == RWSEM_WAITING_BIAS || + (count > RWSEM_WAITING_BIAS && + adjustment != -RWSEM_ACTIVE_READ_BIAS)) sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); raw_spin_unlock_irq(&sem->wait_lock); -- 1.8.1.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/