Le Mon, May 20, 2024 at 11:48:54AM -0700, Paul E. McKenney a écrit :
> 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.
Right but to achieve that, the smp_mb() was already enough, courtesy of
the official full barrier on schedule that (this one at least) we could rely on:
Updater Reader
------ -------
X = 1 rq->curr = A
// another context switch later
smp_mb() smp_mb__after_spin_lock() // right after rq_lock on __schedule()
READ rq->curr rq->curr = B
READ X
If the updater misses A, B will see the update on X.
So I think we still need to justify the rq locking on the comments.
> 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.
That looks good.
>
> I left out the "ordering agains previous tasks accesses" because,
> as you say, this ordering is provided elsewhere.
Right!
Thanks.
