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

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Message-ID: <20131211002330.GA12924@redhat.com>
Date:	Tue, 10 Dec 2013 19:23:30 -0500
From:	Dave Jones <davej@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 07:05:04PM -0500, Steven Rostedt wrote:
 > On Tue, Dec 10, 2013 at 06:00:09PM -0500, Dave Jones wrote:
 > > 
 > > 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 ?
 > 
 > Which thread did not do the futex call? The one that was spinning? No, that one
 > most definitely was, at least according to the stack trace trace you posted:
 > 
 >  trinity-child27-10818 [001] 89790.703547: kernel_stack:         <stack trace>
 > => futex_requeue (ffffffff810df18a)
 > => do_futex (ffffffff810e019e)
 > => SyS_futex (ffffffff810e0de1)
 > => tracesys (ffffffff81760be4)  
 > 
 > It did a futex() system call.
 > 
 > Or are you talking about another thread?

It's the same thread. but here's what it says the last thing it did was..

(gdb) print shm->previous_syscallno[27]
$1 = 288

accept4.

Just to verify I'm looking at the right array member..

(gdb) print shm->pids[27]
$2 = 10818

Oh, hmm. Wait, I'm an idiot.
I only update ->previous when we come back from the syscall.
It's _still_ doing this syscall.  

(gdb) print shm->syscallno[27]
$4 = 202

I was distracted by seeing all the other threads exiting, so I was only looking at
what this one had already done.

ok, mystery solved. derp.

	Dave

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