Re: suspend regression in 4.1-rc1

From: Peter Zijlstra <peterz@infradead.org>
To: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Stephane Eranian <eranian@google.com>,
	Don Zickus <dzickus@redhat.com>, Ingo Molnar <mingo@elte.hu>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Kevin Hilman <khilman@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	linux-pm@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: suspend regression in 4.1-rc1
Date: Mon, 18 May 2015 13:05:13 +0200	[thread overview]
Message-ID: <20150518110513.GE21418@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <309071648.615900.1431946606778.JavaMail.zimbra@redhat.com>

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.