linux-kernel - Re: futex question

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Message-ID: <alpine.LFD.2.00.0910051251080.2646@localhost.localdomain>
Date:	Mon, 5 Oct 2009 12:56:41 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Anirban Sinha <ani@...rban.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Darren Hart <dvhltc@...ibm.com>,
	Kaz Kylheku <kaz@...gmasystems.com>,
	Anirban Sinha <asinha@...gmasystems.com>
Subject: Re: futex question

Peter,

On Mon, 5 Oct 2009, Peter Zijlstra wrote:

> On Sun, 2009-10-04 at 18:59 +0200, Thomas Gleixner wrote:
> 
> > > 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.
> 
> Right, so mm_release() which is meant to destroy the old mm context
> actually does exit_robust_list(), but the problem is that it does so on
> the new mm, not the old one that got passed down to mm_release().
> 
> The other detail is that exit_robust_list() doesn't clear
> current->robust_list.

I know.

> The problem with the patch send my Ani is that it clears the robust
> lists before the point of no return, so on a failing execve() we'd have
> messed up the state.

Right. We need to do that at the latest possible point.

Looking more into that I think we should check whether the robust list
has an entry (lock held) in do_execve() and return -EWOULDBLOCK to
luser space. Same if pi_waiters is not empty. Holding a lock and
calling execve() is simply broken.

Thanks,

	tglx
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