linux-kernel - Re: futex question

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Message-Id: <1254738974.26976.24.camel@twins>
Date:	Mon, 05 Oct 2009 12:36:14 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
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

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.

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.

Making exit_robust_list() deal with an mm that is not the current mm is
interesting indeed.

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