From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934108AbaGXDnX (ORCPT ); Wed, 23 Jul 2014 23:43:23 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:45008 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934076AbaGXDnW (ORCPT ); Wed, 23 Jul 2014 23:43:22 -0400 Date: Wed, 23 Jul 2014 20:43:15 -0700 From: "Paul E. McKenney" To: Pranith Kumar Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value Message-ID: <20140724034315.GJ11241@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1406092194-13004-1-git-send-email-bobby.prani@gmail.com> <1406092194-13004-12-git-send-email-bobby.prani@gmail.com> <20140723122608.GL11241@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14072403-6688-0000-0000-00000384A9D6 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 23, 2014 at 10:36:19PM -0400, Pranith Kumar wrote: > On Wed, Jul 23, 2014 at 8:26 AM, Paul E. McKenney > wrote: > > On Wed, Jul 23, 2014 at 01:09:48AM -0400, Pranith Kumar wrote: > >> When the gp_kthread wakes up from the wait event, it returns 0 if the wake up is > >> due to the condition having been met. This commit checks this return value > >> for a spurious wake up before calling rcu_gp_init(). > >> > >> Signed-off-by: Pranith Kumar > > > > How does this added check help? I don't see that it does. If the flag > > is set, we want to wake up. If we get a spurious wakeup, but then the > > flag gets set before we actually wake up, we still want to wake up. > > So I took a look at the docs again, and using the return value is the > recommended way to check for spurious wakeups. > > The condition in wait_event_interruptible() is checked when the task > is woken up (either due to stray signals or explicitly) and it returns > true if condition evaluates to true. > > In the current scenario, if we get a spurious wakeup, we take the > costly path of checking this condition again (with a barrier and lock) > before going back to wait. > > The scenario of getting an actual wakeup after getting a spurious > wakeup exists even today, this is the window after detecting a > spurious wakeup and before going back to wait. I am not sure if using > the return value enlarges that window as we are going back to sleep > immediately. > > Thoughts? If the flag is set, why should we care whether or not the wakeup was spurious? If the flag is not set, why should we care whether or not wait_event_interruptible() thought that the wakeup was not spurious? Thanx, Paul