On Thu, Sep 16, 2021 at 11:05:14PM +0200, Frederic Weisbecker wrote:
> 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.
I am not claiming that current compilers would mess this up, though I
have learned to have great respect for what future compilers might do...
> 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.
Except that __this_cpu_cmpxchg() would have a possibility of failure,
and thus an extra branch not needed by WRITE_ONCE(). Or am I missing
your point here?
I should hasten to add that getting rid of ->exp_deferred_qs is quite
attractive!
Thanx, Paul
> > > rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
> > > }
> > >
> > > --
> > > 2.25.1
> > >
