linux-kernel - Re: futex question

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Message-ID: <alpine.LFD.2.00.0910041853180.2646@localhost.localdomain>
Date:	Sun, 4 Oct 2009 18:59:46 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Anirban Sinha <ani@...rban.org>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Darren Hart <dvhltc@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Kaz Kylheku <kaz@...gmasystems.com>,
	Anirban Sinha <asinha@...gmasystems.com>
Subject: Re: futex question

On Sun, 4 Oct 2009, Anirban Sinha wrote:

> >> 1) What caused you to instrument this path in the first place?  Were you
> >> seeing some unexpected behavior?
> >>
> >> 2) I wonder why we would need to clear the robust list, but I don't see other
> >> things like pi_blocked_on, etc. in execve being cleared.  I'm looking into
> >> this now (perhaps we don't do the same cleanup, need to check).... have to get
> >> on the plane...
> > 
> > Hmm, just setting the robust list pointer to NULL fixes the problem at
> > hand, but I wonder whether we need to call exit_robust_list() as
> > well. 
 
> hmm. That is an interesting thought. But I wonder if acquiring a
> lock and then exec()ing in the critical section is a legal thing to

It's definitely legal. There is no law which forbids to do that. :)

> do. It does not feel right. Currently, with or without my change,
> such a thing would indefinitely block other waiters on the same
> futex.

Right. Which completely defeats the purpose of the robust list. Will
have a look tomorrow.

Thanks,

	tglx
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