linux-kernel - Re: Random panic in load

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Message-ID: <20140724095517.GS12054@laptop.lan>
Date:	Thu, 24 Jul 2014 11:55:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michel Dänzer <michel@...nzer.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Thu, Jul 24, 2014 at 09:51:57AM +0200, Peter Zijlstra wrote:
> > I hope the assembly output I sent earlier helps, I'm afraid bisecting
> > this could be painful.
> 
> Yeah, lemme go have a look...

So I'm not seeing it, the cpus value is kept at -136(%rbp), so
-128(%rbp) comes after and that's struct lb_env env. And -140(%rbp)
comes before and that ends up being @idle.

The compiler likes to spill for sure, but aside from stupid I don't
see it doing wrong in the relatively short code from function start to
the rep movsl.

It does a rep stosl on -128(%rbp) and then fills it out, but none of
that looks to stomp on our -136(%rbp) value. And the -140(%rbp) thing is
only written to once, and while that is done after the 136 thing its a
single movl and that's not going to clobber anything.

And the fault happens before we pass @env around, so there no chance
someone writes before it either.

So I'm still entirely clueless..
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