linux-kernel - Re: v3.13-rc6+ regression (ARM board)

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Message-ID: <CA+55aFz2dKw5HXy7DV1Aymoscg0MW7Zz28apan8dugUeqTd1QA@mail.gmail.com>
Date:	Thu, 2 Jan 2014 12:43:52 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Krzysztof Hałasa <khalasa@...p.pl>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, Willy Tarreau <w@....eu>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: v3.13-rc6+ regression (ARM board)

On Thu, Jan 2, 2014 at 12:30 PM, John Stultz <john.stultz@...aro.org> wrote:
>
> So something else may be at play. Even with Linus' patch I reproduced a
> similar hang here.
>
> Still chasing it down, but it looks like a seqlock deadlock where we're
> calling read while holding the lock.

Hmm. Only with lockdep, right?

Does lockdep perhaps read the scheduler clock? Afaik, we have
lockstat_clock(), which uses local_clock(), which in turn translates
to sched_clock_cpu(smp_processor_id())..

So if that code now tries to read the scheduler clock when
update_sched_clock() is doing a update and has done a
write_seqcount_begin()...

    Linus
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