linux-kernel - Re: Crashes with 874bbfe600a6 in 3.18.25

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Message-ID: <1454824789.3616.26.camel@gmail.com>
Date:	Sun, 07 Feb 2016 06:59:49 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Tejun Heo <tj@...nel.org>
Cc:	Michal Hocko <mhocko@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
	Ben Hutchings <ben@...adent.org.uk>,
	Sasha Levin <sasha.levin@...cle.com>, Shaohua Li <shli@...com>,
	LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
	Daniel Bilik <daniel.bilik@...system.cz>
Subject: Re: Crashes with 874bbfe600a6 in 3.18.25

On Sun, 2016-02-07 at 06:19 +0100, Mike Galbraith wrote:
> On Sat, 2016-02-06 at 11:07 -0200, Henrique de Moraes Holschuh wrote:
> > On Fri, 05 Feb 2016, Tejun Heo wrote:
> > > On Fri, Feb 05, 2016 at 09:59:49PM +0100, Mike Galbraith wrote:
> > > > On Fri, 2016-02-05 at 15:54 -0500, Tejun Heo wrote:
> > > > 
> > > > > What are you suggesting?
> > > > 
> > > > That 874bbfe6 should die.
> > > 
> > > Yeah, it's gonna be killed.  The commit is there because the behavior
> > > change broke things.  We don't want to guarantee it but have been and
> > > can't change it right away just because we don't like it when things
> > > may break from it.  The plan is to implement a debug option to force
> > > workqueue to always execute these work items on a foreign cpu to weed
> > > out breakages.
> > 
> > Is there a path to filter down sane behavior (whichever one it might be) to
> > the affected stable/LTS kernels?
> 
> What Michal said, replace 874bbfe6 with 176bed1d.  Without 22b886dd,
> 874bbfe6 is a landmine, uses add_timer_on() as if it were mod_timer(),
> which it is not, or rather was not until 22b886dd came along, and still
> does not look like the mod_timer() alias that add_timer() is.

BTW, with the 874bbfe6 22b886dd pair, mundane workqueue timers are no
longer deflected to housekeeper CPUs, so NO_HZ_FULL regresses.

	-Mike