Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755587AbbBPIVZ (ORCPT ); Mon, 16 Feb 2015 03:21:25 -0500 Received: from casper.infradead.org ([85.118.1.10]:54387 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752757AbbBPIVW (ORCPT ); Mon, 16 Feb 2015 03:21:22 -0500 Date: Mon, 16 Feb 2015 09:21:13 +0100 From: Peter Zijlstra To: Oleg Nesterov Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, dave@stgolabs.net, waiman.long@hp.com, raghavendra.kt@linux.vnet.ibm.com, Nicholas Mc Guire , Linus Torvalds , mingo@kernel.org Subject: Re: [PATCH] sched/completion: completion_done() should serialize with complete() Message-ID: <20150216082113.GY5029@twins.programming.kicks-ass.net> References: <20150212003430.GA28656@linux.vnet.ibm.com> <20150212195913.GA30430@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150212195913.GA30430@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3374 Lines: 93 On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote: > Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when > reading completion state" was not correct, without lock/unlock the code > like stop_machine_from_inactive_cpu() > > while (!completion_done()) > cpu_relax(); > > can return before complete() finishes its spin_unlock() which writes to > this memory. And spin_unlock_wait(). > > While at it, change try_wait_for_completion() to use READ_ONCE(). So I share Davidlohrs concern if we should not simply revert that change; but given we've now gone over it detail I suppose we should just keep the optimized version. I did add a comment to your patch; and queued the below for sched/urgent. --- Subject: sched/completion: completion_done() should serialize with complete() From: Oleg Nesterov Date: Thu, 12 Feb 2015 20:59:13 +0100 Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when reading completion state" was not correct, without lock/unlock the code like stop_machine_from_inactive_cpu() while (!completion_done()) cpu_relax(); can return before complete() finishes its spin_unlock() which writes to this memory. And spin_unlock_wait(). While at it, change try_wait_for_completion() to use READ_ONCE(). Fixes: de30ec47302c ("sched/completion: Remove unnecessary ->wait.lock serialization when reading completion state") Cc: waiman.long@hp.com Cc: raghavendra.kt@linux.vnet.ibm.com Cc: dave@stgolabs.net Cc: Nicholas Mc Guire Cc: Linus Torvalds Reported-by: "Paul E. McKenney" Tested-by: "Paul E. McKenney" Reported-by: Davidlohr Bueso Signed-off-by: Oleg Nesterov [peterz: Add a comment with the barrier] Signed-off-by: Peter Zijlstra (Intel) Link: http://lkml.kernel.org/r/20150212195913.GA30430@redhat.com --- kernel/sched/completion.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-) --- a/kernel/sched/completion.c +++ b/kernel/sched/completion.c @@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp * first without taking the lock so we can * return early in the blocking case. */ - if (!ACCESS_ONCE(x->done)) + if (!READ_ONCE(x->done)) return 0; spin_lock_irqsave(&x->wait.lock, flags); @@ -297,6 +297,21 @@ EXPORT_SYMBOL(try_wait_for_completion); */ bool completion_done(struct completion *x) { - return !!ACCESS_ONCE(x->done); + if (!READ_ONCE(x->done)) + return false; + + /* + * If ->done, we need to wait for complete() to release ->wait.lock + * otherwise we can end up freeing the completion before complete() + * is done referencing it. + * + * The RMB pairs with complete()'s RELEASE of ->wait.lock and orders + * the loads of ->done and ->wait.lock such that we cannot observe + * the lock before complete() acquires it while observing the ->done + * after it's acquired the lock. + */ + smp_rmb(); + spin_unlock_wait(&x->wait.lock); + return true; } EXPORT_SYMBOL(completion_done); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/