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

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Date:	Tue, 10 Dec 2013 21:33:30 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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, 10 Dec 2013, 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 ...

The -EAGAIN is when the user value changed, simplified:

    oldval = *uval;
    sys_futex(....., oldval)
      do_futex(...) {		     
	if (oldval != *uval)
	   return -EAGAIN;
      }
    sys_exit();

And user space loops on that.

Thanks,

	tglx
	
	
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