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

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Date:	Wed, 13 Aug 2014 21:57:01 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Oleg Nesterov <oleg@...hat.com>, 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 08/13/2014 08:43 PM, Frederic Weisbecker wrote:
> On Wed, Aug 13, 2014 at 05:03:24PM -0400, Rik van Riel wrote:
>> --- a/kernel/time/posix-cpu-timers.c +++
>> b/kernel/time/posix-cpu-timers.c @@ -272,22 +272,8 @@ static int
>> posix_cpu_clock_get_task(struct task_struct *tsk, if
>> (same_thread_group(tsk, current)) err =
>> cpu_clock_sample(which_clock, tsk, &rtn); } else { -		unsigned
>> long flags; -		struct sighand_struct *sighand; - -		/* -		 *
>> while_each_thread() is not yet entirely RCU safe, -		 * keep
>> locking the group while sampling process -		 * clock for now. -
>> */ -		sighand = lock_task_sighand(tsk, &flags); -		if (!sighand) 
>> -			return err; - if (tsk == current ||
>> thread_group_leader(tsk)) err =
>> cpu_clock_sample_group(which_clock, tsk, &rtn); - -
>> unlock_task_sighand(tsk, &flags); }
> 
> I'm worried about such lockless solution based on RCU or read
> seqcount because we lose the guarantee that an update is
> immediately visible by all subsequent readers.
> 
> Say CPU 0 updates the thread time and both CPU 1 and CPU 2 right
> after that call clock_gettime(), with the spinlock we were
> guaranteed to see the new update. Now with a pure seqlock read
> approach, we guarantee a read sequence coherency but we don't
> guarantee the freshest update result.
> 
> So that looks like a source of non monotonic results.

Which update are you worried about, specifically?

The seq_write_lock to update the usage stat in p->signal will lock out
the seqlock read side used to check those results.

Is there another kind of thing read by cpu_clock_sample_group that you
believe is not excluded by the seq_lock?


- -- 
All rights reversed
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