linux-kernel - Re: process 'stuck' at exit.

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Date:	Tue, 10 Dec 2013 14:58:19 -0800
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: process 'stuck' at exit.

On Tue, 2013-12-10 at 14:48 -0800, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 2:42 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > And yes, I remember that we do not do an extra check for the fshared
> > case, because get_user_pages_fast() should do it for us already. If
> > not we are fubared not only in the futex code.
> 
> Yeah. It turns out we do do the access check indirectly - by looking
> at the PAGE_USER bit, even if we don't necessarily check the actual
> limits. So get_user_pages_fast() is fine.
> 
> > But there is a subtle detail:
> 
> Yup, see my email from ten minutes ago, we found the same thing. And
> that would seem to explain the endless loop, and also the timing
> (since Dave mentions he started doing large-pages lately).
> 
> So I think the "__get_user_pages_fast(address, 1, !ro, &page)" thing
> should work.
> 
> Dave, can you re-create that trinity run and test that patch? I think
> we've got this, but it might be nice to leave the hung machine up and
> running until it's verified.. Although I don't really see what else we
> could need or get out of it, so..

Would it be possible to limit the options to only pass FUTEX_CMP_REQUEUE
and a read-only uaddr? That should improve confidence when it doesn't
fail :-)

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel


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