From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751461AbdGZUSY (ORCPT ); Wed, 26 Jul 2017 16:18:24 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:46176 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751413AbdGZUSX (ORCPT ); Wed, 26 Jul 2017 16:18:23 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 863B460911 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=prsood@codeaurora.org From: Prateek Sood To: peterz@infradead.org, mingo@redhat.com Cc: Prateek Sood , sramana@codeaurora.org, linux-kernel@vger.kernel.org Subject: [PATCH] rwsem: fix missed wakeup due to reordering of load Date: Thu, 27 Jul 2017 01:47:52 +0530 Message-Id: <1501100272-16338-1-git-send-email-prsood@codeaurora.org> X-Mailer: git-send-email 1.9.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If a spinner is present, there is a chance that the load of rwsem_has_spinner() in rwsem_wake() can be reordered with respect to decrement of rwsem count in __up_write() leading to wakeup being missed. spinning writer up_write caller --------------- ----------------------- [S] osq_unlock() [L] osq spin_lock(wait_lock) sem->count=0xFFFFFFFF00000001 +0xFFFFFFFF00000000 count=sem->count MB sem->count=0xFFFFFFFE00000001 -0xFFFFFFFF00000001 spin_trylock(wait_lock) return rwsem_try_write_lock(count) spin_unlock(wait_lock) schedule() Reordering of atomic_long_sub_return_release() in __up_write() and rwsem_has_spinner() in rwsem_wake() can cause missing of wakeup in up_write() context. In spinning writer, sem->count and local variable count is 0XFFFFFFFE00000001. It would result in rwsem_try_write_lock() failing to acquire rwsem and spinning writer going to sleep in rwsem_down_write_failed(). The smp_rmb() will make sure that the spinner state is consulted after sem->count is updated in up_write context. Signed-off-by: Prateek Sood --- kernel/locking/rwsem-xadd.c | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 34e727f..21c111a 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -585,6 +585,40 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) unsigned long flags; DEFINE_WAKE_Q(wake_q); + /* + * If a spinner is present, there is a chance that the load of + * rwsem_has_spinner() in rwsem_wake() can be reordered with + * respect to decrement of rwsem count in __up_write() leading + * to wakeup being missed. + * + * spinning writer up_write caller + * --------------- ----------------------- + * [S] osq_unlock() [L] osq + * spin_lock(wait_lock) + * sem->count=0xFFFFFFFF00000001 + * +0xFFFFFFFF00000000 + * count=sem->count + * MB + * sem->count=0xFFFFFFFE00000001 + * -0xFFFFFFFF00000001 + * spin_trylock(wait_lock) + * return + * rwsem_try_write_lock(count) + * spin_unlock(wait_lock) + * schedule() + * + * Reordering of atomic_long_sub_return_release() in __up_write() + * and rwsem_has_spinner() in rwsem_wake() can cause missing of + * wakeup in up_write() context. In spinning writer, sem->count + * and local variable count is 0XFFFFFFFE00000001. It would result + * in rwsem_try_write_lock() failing to acquire rwsem and spinning + * writer going to sleep in rwsem_down_write_failed(). + * + * The smp_rmb() here is to make sure that the spinner state is + * consulted after sem->count is updated in up_write context. + */ + smp_rmb(); + /* * If a spinner is present, it is not necessary to do the wakeup. * Try to do wakeup only if the trylock succeeds to minimize -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.