Re: [PATCH 2/2] rcu/tasks: Further comment ordering around current task snapshot on TASK-TRACE

[Paul E. McKenney]

On Fri, May 17, 2024 at 05:23:03PM +0200, Frederic Weisbecker wrote:
> Comment the current understanding of barriers and locking role around
> task snapshot.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  kernel/rcu/tasks.h | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 6a9ee35a282e..05413b37dd6e 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1738,9 +1738,21 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
>  	for_each_online_cpu(cpu) {
>  		rcu_read_lock();
>  		/*
> -		 * RQ must be locked because no ordering exists/can be relied upon
> -		 * between rq->curr write and subsequent read sides. This ensures that
> -		 * further context switching tasks will see update side pre-GP accesses.
> +		 * RQ lock + smp_mb__after_spinlock() before reading rq->curr serve
> +		 * two purposes:
> +		 *
> +		 * 1) Ordering against previous tasks accesses (though already enforced
> +		 *    by upcoming IPIs and post-gp synchronize_rcu()).
> +		 *
> +		 * 2) Make sure not to miss latest context switch, because no ordering
> +		 *    exists/can be relied upon between rq->curr write and subsequent read
> +		 *    sides.
> +		 *
> +		 * 3) Make sure subsequent context switching tasks will see update side
> +		 *    pre-GP accesses.
> +		 *
> +		 * smp_mb() after reading rq->curr doesn't play a significant role and might
> +		 * be considered for removal in the future.
>  		 */
>  		t = cpu_curr_snapshot(cpu);
>  		if (rcu_tasks_trace_pertask_prep(t, true))

How about this for that comment?

		// Note that cpu_curr_snapshot() picks up the target
		// CPU's current task while its runqueue is locked with an
		// smp_mb__after_spinlock().  This ensures that subsequent
		// tasks running on that CPU will see the updater's pre-GP
		// accesses.  The trailng smp_mb() in cpu_curr_snapshot()
		// does not currently play a role other than simplify
		// that function's ordering semantics.  If these simplified
		// ordering semantics continue to be redundant, that smp_mb()
		// might be removed.

I left out the "ordering agains previous tasks accesses" because,
as you say, this ordering is provided elsewhere.

Thoughts?

							Thanx, Paul