Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752822AbdHXMdP (ORCPT ); Thu, 24 Aug 2017 08:33:15 -0400 Received: from merlin.infradead.org ([205.233.59.134]:51672 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752563AbdHXMdO (ORCPT ); Thu, 24 Aug 2017 08:33:14 -0400 Date: Thu, 24 Aug 2017 14:33:04 +0200 From: Peter Zijlstra To: Prateek Sood Cc: mingo@redhat.com, sramana@codeaurora.org, linux-kernel@vger.kernel.org, Waiman Long , Davidlohr Bueso , Andrea Parri , Will Deacon Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load Message-ID: <20170824123304.5pqw53qbsytpfbrp@hirez.programming.kicks-ass.net> References: <1503487735-4362-1-git-send-email-prsood@codeaurora.org> <20170824112927.tqejopswi7mcy4sq@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170824112927.tqejopswi7mcy4sq@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2459 Lines: 75 On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote: > > WTH did you not Cc the people that commented on your patch last time? > > On Wed, Aug 23, 2017 at 04:58:55PM +0530, 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 > > RMB > > This doesn't make sense, it appears to order a STORE against something > else. > > > spin_trylock(wait_lock) > > return > > rwsem_try_write_lock(count) > > spin_unlock(wait_lock) > > schedule() Is this what you wanted to write? --- 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 02f660666ab8..813b5d3654ce 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