linux-kernel - Re: regression introduced by - timers: fix itimer/many thread hang

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Date:	Mon, 17 Nov 2008 10:16:55 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Frank Mayhar <fmayhar@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Doug Chapman <doug.chapman@...com>, mingo@...e.hu,
	adobriyan@...il.com, akpm@...ux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression introduced by - timers: fix itimer/many thread hang

> > 	-	if (!->signal)
> > 	+	if (->exit_state)
> > 			return;
> 
> Yes, unless I missed something again, this should work. I'll send
> the (simple) patches soon, but I have no idea how to test them.

That certainly will exclude the problem of crashing in the tick interrupt
after exit_notify.  Unfortunately, it's moving in an undesireable direction
for the long run.  That is, it loses from the accounting even more of the
CPU time spent on the exit path.

> However, I'm afraid there is another problem. On 32 bit cpus we can't
> read "u64 sum_exec_runtime" atomically, and so thread_group_cputime()
> can "overestimate" ->sum_exec_runtime by (up to) UINT_MAX if it races
> with the thread which updates its per_cpu_ptr(.totals). This for example
> means that check_process_timers() can fire the CPUCLOCK_SCHED timers
> before time.
> 
> No?

Yes, I think you're right.  The best solution that comes to mind off hand
is to protect the update/read of that u64 with a seqcount_t on 32-bit.


Thanks,
Roland
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