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

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Message-ID: <alpine.DEB.2.02.1312102133400.28330@ionos.tec.linutronix.de>
Date:	Tue, 10 Dec 2013 21:34:24 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Jones <davej@...hat.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, 10 Dec 2013, Dave Jones wrote:
> On Tue, Dec 10, 2013 at 11:55:06AM -0800, Linus Torvalds wrote:
>  > 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.
> 
> btw, I've left the machine in that state, and will for as long as necesary
> in case someone has any ideas for further tracing experiments.

Can you gather a trace with the function tracer? That will tell us
what the thing is actually doing.

Thanks,

	tglx
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