linux-kernel - Re: [PATCH] futex: fix a race condition between REQUEUE

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Message-ID: <alpine.DEB.2.11.1410261528270.5308@nanos>
Date:	Sun, 26 Oct 2014 15:29:06 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Brian Silverman <bsilver16384@...il.com>
cc:	Austin Schuh <austin.linux@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] futex: fix a race condition between REQUEUE_PI and task
 death

On Sat, 25 Oct 2014, Brian Silverman wrote:
> On Sat, 25 Oct 2014, Thomas Gleixner wrote:
> 
> > > pi_state_free and exit_pi_state_list both clean up futex_pi_state's.
> > > exit_pi_state_list takes the hb lock first, and most callers of
> > > pi_state_free do too. requeue_pi didn't, which causes lots of problems.
> >
> > "causes lots of problems" is not really a good explanation of the root
> > cause. That wants a proper description of the race, i.e.
> >
> > CPU 0              CPU 1
> > ...                ....
> >
> > I'm surely someone who is familiar with that code, but it took me
> > quite some time to understand whats going on. The casual reader will
> > just go into brain spiral mode and give up.
> 
> Thinking about it again, I'm no longer so sure that exit_pi_state_list is the
> only place that it can race against. However, I did catch that one with a
> particularly lucky crashdump, so I know it _can_ happen there. Is just
> giving an example for that good?

Sure. That immediately makes clear whats going on.

Thanks,

	tglx
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