linux-kernel - Re: trinity finds ftrace/perf bug. Film at 11.

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Message-ID: <20131003154401.GO31370@twins.programming.kicks-ass.net>
Date:	Thu, 3 Oct 2013 17:44:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dave Jones <davej@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>, jolsa@...hat.com
Subject: Re: trinity finds ftrace/perf bug. Film at 11.

On Thu, Sep 12, 2013 at 02:38:49PM -0400, Dave Jones wrote:

> The current one I'm staring at is this from LIST_DEBUG..
> 
> list_del corruption. next->prev should be prev (ffff88000fb812b0), but was ffff88000fb812b0. (next=ffff88009df8b7b0).
> 
> The sharp eyed will notice that those first two addresses are actually the same.
> So we matched the next->prev != prev test, but at the time we got to the printk that follows
> in the WARN, the inverse was true. Smells like a race of some kind.

FWIW, I've not given up on this. I've found one bug in the list_entry
handling already -- although not this one.

Today I hit a more revealing error:

[ 8348.150303] list_del corruption. prev->next should be ffff88042839c108, but was 6b6b6b6b6b6b6b6b

And as we all know that 6b is POISON_FREE, so we're staring at a
use-after-free here.

Brain did give out for today though.. more staring at reference counts
tomorrow.

The sad thing is that reproduction takes forever on my machine; I need
to let trinity run for a good hour and then ^C abort the run. Only then
will I hit it fairly reliably.
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