From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752301AbbCGC4g (ORCPT <rfc822;w@1wt.eu>);
	Fri, 6 Mar 2015 21:56:36 -0500
Received: from g4t3427.houston.hp.com ([15.201.208.55]:53661 "EHLO
	g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750698AbbCGC4f (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 6 Mar 2015 21:56:35 -0500
Message-ID: <1425696991.2475.329.camel@j-VirtualBox>
Subject: Re: softlockups in multi_cpu_stop
From: Jason Low <jason.low2@hp.com>
To: Ming Lei <ming.lei@canonical.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Sasha Levin <sasha.levin@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>, jason.low2@hp.com
Date: Fri, 06 Mar 2015 18:56:31 -0800
In-Reply-To: <CACVXFVPk9Jp1vaf12xSc4cpypbYd_3bcSVkKtarSpoiPNK_MNg@mail.gmail.com>
References: <54F41516.6060608@oracle.com> <54F98F1F.3080107@oracle.com>
	 <20150306123233.GA9972@gmail.com> <1425662342.19505.41.camel@stgolabs.net>
	 <1425668223.2475.94.camel@j-VirtualBox>
	 <CA+55aFwZufdV5Q7Wm+b2F8KurtgXsJ_eNe9b6_TOSUhuW_GfSg@mail.gmail.com>
	 <1425670188.2475.113.camel@j-VirtualBox>
	 <1425676346.2475.135.camel@j-VirtualBox>
	 <1425680137.19505.63.camel@stgolabs.net>
	 <20150307095526.5ffb1bf0@tom-ThinkPad-T410>
	 <1425694046.19505.71.camel@stgolabs.net>
	 <CACVXFVPk9Jp1vaf12xSc4cpypbYd_3bcSVkKtarSpoiPNK_MNg@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
X-Mailer: Evolution 3.2.3-0ubuntu6 
Content-Transfer-Encoding: 7bit
Mime-Version: 1.0
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sat, 2015-03-07 at 10:10 +0800, Ming Lei wrote:
> On Sat, Mar 7, 2015 at 10:07 AM, Davidlohr Bueso <dave@stgolabs.net> wrote:
> > On Sat, 2015-03-07 at 09:55 +0800, Ming Lei wrote:
> >> On Fri, 06 Mar 2015 14:15:37 -0800
> >> Davidlohr Bueso <dave@stgolabs.net> wrote:
> >>
> >> > On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote:
> >> > > In owner_running() there are 2 conditions that would make it return
> >> > > false: if the owner changed or if the owner is not running. However,
> >> > > that patch continues spinning if there is a "new owner" but it does not
> >> > > take into account that we may want to stop spinning if the owner is not
> >> > > running (due to getting rescheduled).
> >> >
> >> > So you're rationale is that we're missing this need_resched:
> >> >
> >> >     while (owner_running(sem, owner)) {
> >> >             /* abort spinning when need_resched */
> >> >             if (need_resched()) {
> >> >                     rcu_read_unlock();
> >> >                     return false;
> >> >             }
> >> >     }
> >> >
> >> > Because the owner_running() would return false, right? Yeah that makes
> >> > sense, as missing a resched is a bug, as opposed to our heuristics being
> >> > so painfully off.
> >> >
> >> > Sasha, Ming (Cc'ed), does this address the issues you guys are seeing?
> >>
> >> For the xfstest lockup, what matters is that the owner isn't running, since
> >> the following simple change does fix the issue:
> >
> > I much prefer Jason's approach, which should also take care of the
> > issue, as it includes the !owner->on_cpu stop condition to stop
> > spinning.
> 
> But the check on owner->on_cpu should be moved outside the loop
> because new owner can be scheduled out too, right?

We should keep the owner->on_cpu check inside the loop, otherwise we
could continue spinning if the owner is not running.