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

Frederic Weisbecker
2014-08-14T00:43:25Z

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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