linux-kernel - Re: suspend regression in 4.1-rc1

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Message-ID: <20150519172020.GA28241@dhcp22.suse.cz>
Date:	Tue, 19 May 2015 19:20:21 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Ulrich Obergfell <uobergfe@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stephane Eranian <eranian@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...nel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>, linux-pm@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: suspend regression in 4.1-rc1

On Mon 18-05-15 11:45:31, Don Zickus wrote:
> On Mon, May 18, 2015 at 04:41:37PM +0200, Michal Hocko wrote:
> > On Mon 18-05-15 10:26:07, Don Zickus wrote:
> > > On Mon, May 18, 2015 at 06:56:46AM -0400, Ulrich Obergfell wrote:
> > > > 
> > > > > There further appears to be a distinct lack of serialization between
> > > > > setting and using watchdog_enabled, so perhaps we should wrap the
> > > > > {en,dis}able_all() things in watchdog_proc_mutex.
> > > > 
> > > > As I understand it, the {en,dis}able_all() functions are only called early
> > > > at kernel startup, so I do not see how they could be racing with watchdog
> > > > code that is executed in the context of write() system calls to parameters
> > > > in /proc/sys/kernel. Please see also my earlier reply to Michal for further
> > > > details: http://marc.info/?l=linux-pm&m=143194387208250&w=2
> > > > 
> > > > Do we really need synchronization here?
> > > 
> > > As Peter said we have to focus on doing things correctly and not based on
> > > what is currently.
> > > 
> > > During s2ram, I believe all the threads get parked and then unparked during
> > > resume.  I am wondering if the race happens there, threads get unparked and
> > > stomp on each other when watchdog_nmi_enable_all() is called.
> > 
> > Wouldn't that cause an issue during freezer mode of pm_test? I can see
> > it much later in the processors mode.
> 
> I am not familiar with the freeze mode of pm_test. 

AFAIU only suspend_prepare is called for this mode. This will trigger
pm_notifier_call_chain(PM_SUSPEND_PREPARE) and suspend_freeze_processes
which will freeze all the tasks (including kernel threads).

The hang happened with processors mode which means that everything up to
platform mode was OK. Which reduces the area to disable_nonboot_cpus.

I have tried to put some printks around to see where things go wrong but
then I found out that my netconsole doesn't dump them because the
network cards are long suspended. no_console_suspend apparently doesn't
affect netconsole and the laptop doesn't have regular serial console so
I just gave up.

-- 
Michal Hocko
SUSE Labs
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