linux-kernel - Re: [patch 1/4] rtmutex: Prevent dequeue vs. unlock race

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Date:   Thu, 1 Dec 2016 19:25:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        David Daney <ddaney@...iumnetworks.com>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>, stable@...r.kernel.org
Subject: Re: [patch 1/4] rtmutex: Prevent dequeue vs. unlock race

On Wed, Nov 30, 2016 at 09:04:41PM -0000, Thomas Gleixner wrote:
> It's remarkable that the test program provided by David triggers on ARM64
> and MIPS64 really quick, but it refuses to reproduce on x8664, while the
> problem exists there as well. That refusal might explain that this got not
> discovered earlier despite the bug existing from day one of the rtmutex
> implementation more than 10 years ago.

> -		clear_rt_mutex_waiters(lock);

So that compiles into:

	andq   $0xfffffffffffffffe,0x48(%rbx)

With is a RmW memop. Now per the architecture documents we can decompose
that into a normal load-store and the race exists. But I would not be
surprised if that starts with the cacheline in exclusive mode (because
it knows it will do the store). Which makes it a very tiny race indeed.