On Thu, Sep 16, 2021 at 09:43:40AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 16, 2021 at 02:10:46PM +0200, Frederic Weisbecker wrote:
> > This variable is never written nor read remotely. Remove this confusion.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > ---
> > kernel/rcu/tree_exp.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index f3947c49eee7..4266610b4587 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -255,7 +255,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
> > */
> > static void rcu_report_exp_rdp(struct rcu_data *rdp)
> > {
> > - WRITE_ONCE(rdp->exp_deferred_qs, false);
> > + rdp->exp_deferred_qs = false;
>
> Are you sure that this can never be invoked from an interrupt handler?
> And that rdp->exp_deferred_qs is never read from an interrupt handler?
> If either can happen, then the WRITE_ONCE() does play a role, right?
Well, the only effect I can imagine is that it can partly prevent from an
interrupt to report concurrently the quiescent state during the few
instructions before we mask interrupts and lock the node.
That's a micro performance benefit that avoid a second call to
rcu_report_exp_cpu_mult() with the extra locking and early exit.
But then that racy interrupt can still happen before we clear exp_deferred_qs.
In this case __this_cpu_cmpxchg() would have been more efficient.
Thanks.
> Thanx, Paul
>
> > rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
> > }
> >
> > --
> > 2.25.1
> >
