From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752943AbcELW5Q (ORCPT ); Thu, 12 May 2016 18:57:16 -0400 Received: from g2t4622.austin.hp.com ([15.73.212.79]:42951 "EHLO g2t4622.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752239AbcELW5N (ORCPT ); Thu, 12 May 2016 18:57:13 -0400 From: Waiman Long To: Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, Davidlohr Bueso , Jason Low , Dave Chinner , Peter Hurley , Scott J Norton , Douglas Hatch , Waiman Long Subject: [PATCH v3 3/4] locking/rwsem: Improve reader wakeup code Date: Thu, 12 May 2016 18:56:50 -0400 Message-Id: <1463093811-10684-4-git-send-email-Waiman.Long@hpe.com> X-Mailer: git-send-email 1.7.1 In-Reply-To: <1463093811-10684-1-git-send-email-Waiman.Long@hpe.com> References: <1463093811-10684-1-git-send-email-Waiman.Long@hpe.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In __rwsem_do_wake(), the reader wakeup code will assume a writer has stolen the lock if the active reader/writer count is not 0. However, this is not as reliable an indicator as the original "< RWSEM_WAITING_BIAS" check. If another reader is present, the code will still break out and exit even if the writer is gone. This patch changes it to check the same "< RWSEM_WAITING_BIAS" condition to reduce the chance of false positive. Signed-off-by: Waiman Long --- kernel/locking/rwsem-xadd.c | 11 ++++++++--- 1 files changed, 8 insertions(+), 3 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 22f7d58..6c08ad9 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -148,9 +148,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) try_reader_grant: oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { - /* A writer stole the lock. Undo our reader grant. */ - if (rwsem_atomic_update(-adjustment, sem) & - RWSEM_ACTIVE_MASK) + /* + * If the count is still less than RWSEM_WAITING_BIAS + * after removing the adjustment, it is assumed that + * a writer has stolen the lock. We have to undo our + * reader grant. + */ + if (rwsem_atomic_update(-adjustment, sem) + < RWSEM_WAITING_BIAS) goto out; /* Last active locker left. Retry waking readers. */ goto try_reader_grant; -- 1.7.1