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

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Message-ID: <CA+55aFzLnheC6W+Db=-MVr6aR5UzP-c1Q1yk_aN-9gxSjUpANw@mail.gmail.com>
Date:	Tue, 10 Dec 2013 11:55:06 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Dave Jones <davej@...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>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: process 'stuck' at exit.

On Tue, Dec 10, 2013 at 11:18 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> So this is pretty unlikely. The retry requires:
>
>    get_futex_value_locked() == EFAULT;
>
> Now we drop the hash bucket locks and do:
>
>    get_user();
>
> And if that get_user() faults again, we bail out.

I think you need to look closer.

We have at least also that "futex_proxy_trylock_atomic() returns
-EAGAIN" case. Which triggers at some exit condition. Another thread
in the same group, perhaps never completing the exit because it's
waiting for this one? I dunno, I didn't look any closer (but this does
make me think "Hey, we should add Oleg to the Cc too", since
PF_EXITING is involved).. So maybe there is some situation where that
EAGAIN will keep happening, forever..

Now, I'm *not* saying that that is it. It's quite possible/likely some
other loop, but I do have to say that it sure isn't _obvious_. And
that whole EAGAIN return case is quite deep and special, so ...

                 Linus

PS: Oleg - the whole thread is on lkml. Ping me if you need more context.
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