On Mon, May 20, 2024 at 10:41:52PM +0200, Frederic Weisbecker wrote:
> 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!
Good points! How about the following?
// 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 either
// the grace-period kthread will see that task's read-side
// critical section or the task 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.
Keeping in mind that the commit's log fully lays out the troublesome
scenario.
Thanx, Paul
