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

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Date:	Thu, 14 Aug 2014 02:43:21 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Rik van Riel <riel@...hat.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

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.

>  
>  	if (!err)
> --
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