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

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Date:	Mon, 18 May 2015 12:51:44 +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


Trim emails already.. this seems a spreading disease.

On Mon, May 18, 2015 at 06:10:20AM -0400, Ulrich Obergfell wrote:
> Michal,
> 
> if I understand you correctly, Peter's patch solves the problem for you.
> I would like to make you aware of a patch that Don and I posted in April.
> 
>   https://lkml.org/lkml/2015/4/22/306
> 
> watchdog_nmi_enable_all() should not use 'watchdog_user_enabled' at all.
> It should rather check the NMI_WATCHDOG_ENABLED bit in 'watchdog_enabled'.
> The patch is also in Andrew Morton's queue.
> 
>   http://ozlabs.org/~akpm/mmots/broken-out/watchdog-fix-watchdog_nmi_enable_all.patch
> 
> Peter's patch introduces the same change in watchdog_nmi_enable_all(),
> plus some synchronization. However, I'm not sure if we actually need the
> synchronization. It is my understanding that {en,dis}able_all() are only
> called early during kernel startup via initcall 'fixup_ht_bug':
> 
>   kernel_init
>   {
>     kernel_init_freeable
>     {
>       lockup_detector_init
>       {
>         watchdog_enable_all_cpus
>           smpboot_register_percpu_thread(&watchdog_threads)
>       }
> 
>       do_basic_setup
>         do_initcalls
>           do_initcall_level
>             do_one_initcall
>               fixup_ht_bug // subsys_initcall(fixup_ht_bug)
>               {
>                 watchdog_nmi_disable_all
> 
>                 watchdog_nmi_enable_all
>               }
>     }
>   }
> 
> Peter,
> 
> do we really need the synchronization here?

Well, those are the only current usage sites, but the interface is
exposed and should be fully and correctly implemented, otherwise a next
user might stumble upon sudden unexpected behaviour.

But yes, it appears superfluous for this particular usage.
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