From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752842AbdHQPHr (ORCPT ); Thu, 17 Aug 2017 11:07:47 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:53074 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751882AbdHQPHp (ORCPT ); Thu, 17 Aug 2017 11:07:45 -0400 Date: Thu, 17 Aug 2017 08:07:36 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, Ingo Molnar , Will Deacon , Alan Stern , Andrea Parri , Linus Torvalds Subject: Re: [PATCH v5 tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair Reply-To: paulmck@linux.vnet.ibm.com References: <20170724221252.GA14238@linux.vnet.ibm.com> <1500934389-14942-4-git-send-email-paulmck@linux.vnet.ibm.com> <20170815161629.GA14379@linux.vnet.ibm.com> <20170816112235.3acc59f2@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170816112235.3acc59f2@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17081715-0040-0000-0000-000003911FC6 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007561; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000222; SDB=6.00903666; UDB=6.00452712; IPR=6.00683878; BA=6.00005538; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016741; XFM=3.00000015; UTC=2017-08-17 15:07:42 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17081715-0041-0000-0000-000007854EFE Message-Id: <20170817150736.GQ7017@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-08-17_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1708170250 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 16, 2017 at 11:22:35AM -0400, Steven Rostedt wrote: > On Tue, 15 Aug 2017 09:16:29 -0700 > "Paul E. McKenney" wrote: > > > There is no agreed-upon definition of spin_unlock_wait()'s semantics, > > and it appears that all callers could do just as well with a lock/unlock > > pair. This commit therefore replaces the spin_unlock_wait() call in > > completion_done() with spin_lock() followed immediately by spin_unlock(). > > This should be safe from a performance perspective because the lock > > will be held only the wakeup happens really quickly. > > > > Signed-off-by: Paul E. McKenney > > Cc: Ingo Molnar > > Cc: Peter Zijlstra > > Cc: Will Deacon > > Cc: Alan Stern > > Cc: Andrea Parri > > Cc: Linus Torvalds > > [ paulmck: Updated to use irqsave based on 0day Test Robot feedback. ] > > > > diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c > > index 13fc5ae9bf2f..c9524d2d9316 100644 > > --- a/kernel/sched/completion.c > > +++ b/kernel/sched/completion.c > > @@ -300,6 +300,8 @@ EXPORT_SYMBOL(try_wait_for_completion); > > */ > > bool completion_done(struct completion *x) > > { > > + unsigned long flags; > > + > > if (!READ_ONCE(x->done)) > > return false; > > > > @@ -307,14 +309,9 @@ bool completion_done(struct completion *x) > > * 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); > > + spin_lock_irqsave(&x->wait.lock, flags); > > + spin_unlock_irqrestore(&x->wait.lock, flags); > > return true; > > } > > EXPORT_SYMBOL(completion_done); > > For this patch: > > Reviewed-by: Steven Rostedt (VMware) Applied, thank you! > But I was looking at this function, and it is a little worrisome, as it > says it should return false if there are waiters and true otherwise. > But it can also return false if there are no waiters and the completion > is already done. > > Basically we have: > > wait_for_completion() { > while (!done) > wait(); > done--; > } > > complete() { > done++; > wake_up_waiters(); > } > > Thus, completion_done() only returns true if a complete happened and a > wait_for_completion has not. It does not return true if the complete > has not yet occurred, but there are still waiters. > > I looked at a couple of use cases, and this does not appear to be an > issue, but the documentation about the completion_done() does not > exactly fit the implementation. Should that be addressed? > > Also, if complete_all() is called, then reinit_completion() must be > called before that completion is used. The reinit_completion() has a > comment stating this, but there's no comment by complete_all() stating > this, which is where it really should be. I'll send a patch to fix this > one. But I am too late to return the favor -- good patch, though! Thanx, Paul