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

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Date:	Mon, 18 May 2015 13:05:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ulrich Obergfell <uobergfe@...hat.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stephane Eranian <eranian@...gle.com>,
	Don Zickus <dzickus@...hat.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, May 18, 2015 at 06:56:46AM -0400, Ulrich Obergfell wrote:
> > Subject: watchdog: Fix merge 'conflict'
> >
> > Two watchdog changes that came through different trees had a non
> > conflicting conflict, that is, one changed the semantics of a variable
> > but no actual code conflict happened. So the merge appeared fine, but
> > the resulting code did not behave as expected.
> >
> > Commit 195daf665a62 ("watchdog: enable the new user interface of the
> > watchdog mechanism") changes the semantics of watchdog_user_enabled,
> > which thereafter is only used by the functions introduced by
> > b3738d293233 ("watchdog: Add watchdog enable/disable all functions").
> 
> Don and I already posted a patch in April to address this:
> 
> https://lkml.org/lkml/2015/4/22/306
> http://ozlabs.org/~akpm/mmots/broken-out/watchdog-fix-watchdog_nmi_enable_all.patch

Yeah, but it seems to have gotten lost on its way to Linus.

> > 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?

Same argument as in my previous email; its best to implement exposed
functions fully and correctly, irrespective of their usage sites.

It costs little extra and might safe a few hairs down the lined. None of
this is performance critical.
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