Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S970021AbdIZShq (ORCPT ); Tue, 26 Sep 2017 14:37:46 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:40368 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965144AbdIZSho (ORCPT ); Tue, 26 Sep 2017 14:37:44 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 02FD960134 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 Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load To: mingo@kernel.org, longman@redhat.com, peterz@infradead.org, parri.andrea@gmail.com, dave@stgolabs.net Cc: linux-kernel@vger.kernel.org, sramana@codeaurora.org References: <1504794658-15397-1-git-send-email-prsood@codeaurora.org> From: Prateek Sood Message-ID: Date: Wed, 27 Sep 2017 00:07:38 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <1504794658-15397-1-git-send-email-prsood@codeaurora.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3045 Lines: 89 On 09/07/2017 08:00 PM, Prateek Sood wrote: > 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 | 27 +++++++++++++++++++++++++++ > 1 file changed, 27 insertions(+) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index 02f6606..1fefe6d 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) > DEFINE_WAKE_Q(wake_q); > > /* > + * __rwsem_down_write_failed_common(sem) > + * rwsem_optimistic_spin(sem) > + * osq_unlock(sem->osq) > + * ... > + * atomic_long_add_return(&sem->count) > + * > + * - VS - > + * > + * __up_write() > + * if (atomic_long_sub_return_release(&sem->count) < 0) > + * rwsem_wake(sem) > + * osq_is_locked(&sem->osq) > + * > + * And __up_write() must observe !osq_is_locked() when it observes the > + * atomic_long_add_return() in order to not miss a wakeup. > + * > + * This boils down to: > + * > + * [S.rel] X = 1 [RmW] r0 = (Y += 0) > + * MB RMB > + * [RmW] Y += 1 [L] r1 = X > + * > + * exists (r0=1 /\ r1=0) > + */ > + 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 > * spinlock contention which may introduce too much delay in the > Hi Folks, Do you have any more suggestion/feedback on this patch. Regards Prateek -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project