From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: [PATCH v5 9/9] locking/rwsem: Enable reader lock stealing Date: Thu, 1 Jun 2017 13:39:07 -0400 Message-ID: <1496338747-20398-10-git-send-email-longman@redhat.com> References: <1496338747-20398-1-git-send-email-longman@redhat.com> Return-path: In-Reply-To: <1496338747-20398-1-git-send-email-longman@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Archive: List-Post: To: Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Davidlohr Bueso , Dave Chinner , Waiman Long List-ID: The rwsem has supported writer lock stealing for a long time. Reader lock stealing isn't allowed as it may lead to writer lock starvation. As a result, writers are preferred over readers. However, preferring readers generally leads to better overall performance. This patch now enables reader lock stealing on a rwsem as long as the lock is reader-owned and optimistic spinning hasn't been disabled because of long writer wait. This will improve overall performance without running the risk of writer lock starvation. Signed-off-by: Waiman Long --- kernel/locking/rwsem-xadd.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index a571bec..f5caba8 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -529,6 +529,14 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) goto enqueue; /* + * Steal the lock if no writer was present and the optimistic + * spinning disable bit isn't set. + */ + count = atomic_long_read(&sem->count); + if (!count_has_writer(count)) + return sem; + + /* * Undo read bias from down_read operation to stop active locking if: * 1) Optimistic spinners are present; * 2) the wait_lock isn't free; or -- 1.8.3.1