Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754348AbbBMVJZ (ORCPT ); Fri, 13 Feb 2015 16:09:25 -0500 Received: from e38.co.us.ibm.com ([32.97.110.159]:47663 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754289AbbBMVJV (ORCPT ); Fri, 13 Feb 2015 16:09:21 -0500 Date: Fri, 13 Feb 2015 13:09:14 -0800 From: "Paul E. McKenney" To: Oleg Nesterov Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, dave@stgolabs.net, waiman.long@hp.com, raghavendra.kt@linux.vnet.ibm.com Subject: Re: [PATCH] sched/completion: completion_done() should serialize with complete() Message-ID: <20150213210913.GZ4166@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15021321-0029-0000-0000-000007E835D1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1743 Lines: 53 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(). > > Reported-by: "Paul E. McKenney" > Reported-by: Davidlohr Bueso > Signed-off-by: Oleg Nesterov So I am having some difficulty reproducing the original problem, but the patch passes rcutorture testing. So... Tested-by: Paul E. McKenney > --- x/kernel/sched/completion.c > +++ x/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 +297,11 @@ EXPORT_SYMBOL(try_wait_for_completion); > */ > bool completion_done(struct completion *x) > { > - return !!ACCESS_ONCE(x->done); > + if (!READ_ONCE(x->done)) > + return false; > + > + 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/