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

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Message-ID: <20131210230009.GF5050@redhat.com>
Date:	Tue, 10 Dec 2013 18:00:09 -0500
From:	Dave Jones <davej@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Darren Hart <dvhart@...ux.intel.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, Dec 10, 2013 at 02:48:52PM -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..

The only thing I'm still unclear on, is how that pid allegedly wasn't doing
a futex call as part of its run. The only thing I can think of is that
the other pid that _did_ do a futex call did it on a page that was MAP_SHARED
between all the other children, and this 'spin forever' thing only
happens when the last process with a reference on that page exits ?

does that make sense?

	Dave

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