linux-kernel - Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

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Message-ID: <53ECD7C8.6040202@redhat.com>
Date:	Thu, 14 Aug 2014 11:37:44 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
 seqlock

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On 08/14/2014 10:24 AM, Oleg Nesterov wrote:
> On 08/13, Rik van Riel wrote:
>> 
>> @@ -862,11 +862,9 @@ void do_sys_times(struct tms *tms) { 
>> cputime_t tgutime, tgstime, cutime, cstime;
>> 
>> -	spin_lock_irq(&current->sighand->siglock); 
>> thread_group_cputime_adjusted(current, &tgutime, &tgstime); 
>> cutime = current->signal->cutime; cstime =
>> current->signal->cstime; -
>> spin_unlock_irq(&current->sighand->siglock);
> 
> Ah, wait, there is another problem afaics...

Last night I worked on another problem with this code.

After propagating the stats from a dying task to the signal struct,
we need to make sure that that task's stats are not counted twice.

This requires zeroing the stats under the write_seqlock, which was
easy enough to add. We cannot rely on any state in the task that
was set outside of the write_seqlock...

> thread_group_cputime_adjusted()->cputime_adjust() plays with 
> signal->prev_cputime and thus it needs siglock or stats_lock to
> ensure it can't race with itself. Not sure it is safe to simply
> take the lock in cputime_adjust(), this should be checked.
> 
> OTOH, do_task_stat() already calls task_cputime_adjusted() lockless
> and this looks wrong or I missed something. So perhaps we need a
> lock in or around cputime_adjust() anyway.

I'll take a look at this.

- -- 
All rights reversed
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