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

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Date:	Mon, 17 Nov 2008 13:49:46 -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

> But please note that currently this already happens for sub-threads (and
> if we protect ->signal with rcu too), the exiting sub-thread does not
> contribute to accounting after release_task(self). Also, when the last
> thread exits the process can be reaped by its parent, but after that
> the threads can still use CPU.

Understood (I think I mentioned this earlier).  All this is what makes it
seem potentially attractive in the long run to reorganize this more thoroughly.

> > 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.
> 
> Oh, but we need them to be per-cpu, and both read and write need memory
> barriers... Not that I argue, this will fix the problem of course, just
> I don't know how this impacts the perfomance.

I agree it's a sticky question.  Just the only thing I've thought of so far.


Thanks,
Roland
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