From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758607AbaGXCgx (ORCPT ); Wed, 23 Jul 2014 22:36:53 -0400 Received: from mail-la0-f41.google.com ([209.85.215.41]:52676 "EHLO mail-la0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757538AbaGXCgw (ORCPT ); Wed, 23 Jul 2014 22:36:52 -0400 MIME-Version: 1.0 In-Reply-To: <20140723122608.GL11241@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> From: Pranith Kumar Date: Wed, 23 Jul 2014 22:36:19 -0400 Message-ID: Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value To: Paul McKenney Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? -- Pranith