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

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Message-ID: <20140102101455.GG10158@pengutronix.de>
Date:	Thu, 2 Jan 2014 11:14:55 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Krzysztof Hałasa <khalasa@...p.pl>
Cc:	Willy Tarreau <w@....eu>, John Stultz <john.stultz@...aro.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: v3.13-rc6+ regression (ARM board)

Hello Krzysztof,

On Thu, Jan 02, 2014 at 11:02:44AM +0100, Krzysztof Hałasa wrote:
> >> There seems to be a regression in v3.13-rc6+ (up to current tip =
> >> 71ce176ee6ed1735b9a1160a5704a915d13849b1).
> >>
> >> Board is Gateworks Cambria, CPU Intel IXP435 ARM big endian, gcc 4.7.3.
> >> The board boots correctly and works (shell mostly, and SSHD) for about
> >> 50 seconds. After 52-54 seconds, it frozes dead without any console
> >> (UART) output.
> >>
> >> Bisecting shows 5e30025a319910695f5010dc0fb53a23299da14d as the first
> >> bad commit. Interestingly it's a merge:
> 
> Reverting 1ca7d67cf5d5a2aef26a8d9afd789006fa098347 on top of current tip
> (9a0bb2966efbf30a71c128c3af63307d8b5f5fc0) fixes my issue.
> 
> What now?
> 
> 1ca7d67cf5d5a2aef26a8d9afd789006fa098347:
>     seqcount: Add lockdep functionality to seqcount/seqlock structures
> 
>     Currently seqlocks and seqcounts don't support lockdep.
> 
>     After running across a seqcount related deadlock in the timekeeping
>     code, I used a less-refined and more focused variant of this patch
>     to narrow down the cause of the issue.
> 
>     This is a first-pass attempt to properly enable lockdep functionality
>     on seqlocks and seqcounts.
> 
>     Since seqcounts are used in the vdso gettimeofday code, I've provided
>     non-lockdep accessors for those needs.
> 
>     I've also handled one case where there were nested seqlock writers
>     and there may be more edge cases.
> 
>     Comments and feedback would be appreciated!
So it seems to be that 1ca7d67cf5d5a2aef26a8d9afd789006fa098347 doesn't
like to be combined with some other commit from
5e30025a319910695f5010dc0fb53a23299da14d^2..5e30025a319910695f5010dc0fb53a23299da14d^.
What you can do is:

	git bisect start 5e30025a319910695f5010dc0fb53a23299da14d^ b8a216269ec0ce2e961d32e6d640d7010b8a818e

The bad commit is the tip of Linus' tree when he created the bad merge
commit. The good one is the base of the merged tree. To check if a tree
is good or bad do:

	git merge --no-commit 5e30025a319910695f5010dc0fb53a23299da14d^2
	test test test
	git reset --hard
	git bisect good ... or ... git bisect bad

That should find the commit that makes your board break in combination
with 1ca7d67cf5d5a2aef26a8d9afd789006fa098347.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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