From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751305AbdHQI0W (ORCPT ); Thu, 17 Aug 2017 04:26:22 -0400 Received: from mail-wr0-f196.google.com ([209.85.128.196]:37488 "EHLO mail-wr0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750755AbdHQI0U (ORCPT ); Thu, 17 Aug 2017 04:26:20 -0400 Date: Thu, 17 Aug 2017 10:26:16 +0200 From: Ingo Molnar To: "Paul E. McKenney" Cc: linux-kernel@vger.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, rostedt@goodmis.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 Message-ID: <20170817082616.t34xbzbpdxd2lye2@gmail.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170815161629.GA14379@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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); I'm fine with this patch - as long as there are no performance regression reports. (which I suspect there won't be.) Would you like to carry this in the RCU tree, due to other changes depending on this change - or can I pick this up into the scheduler tree? Thanks, Ingo