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

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Message-ID: <CA+55aFwr2e5x13fA+QWjt=iJFmCcugFErdqaeeUtH4XPBExC6Q@mail.gmail.com>
Date:	Tue, 10 Dec 2013 13:57:49 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 1:41 PM, Dave Jones <davej@...hat.com> wrote:
>
> http://codemonkey.org.uk/junk/trace

Hmm. Ok, so something is calling [__]get_user_pages_fast() and
put_page() in a loop, but the trace doesn't show what that "something"
is, because it is itself not ever called.

However, that pattern does seem to imply that the loop is in
get_futex_key(), because all the other loops I see seem to be calling
other things as well.

And the __get_user_pages_fast() call implies that it's the THP case
that triggers the "unlikely(PageTail(page))" case. And anyway,
otherwise we'd see lock_page()/unlock_page() too.

So it looks like __get_user_pages_fast() fails, and keeps failing.
Andrea, this is your code, any ideas? Commit a5b338f2b0b1f ("thp:
update futex compound knowledge") to be exact.

                   Linus
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