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SubjectRe: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree
On Thu, Mar 21, 2019 at 09:27:37AM +0100, Sebastian Andrzej Siewior wrote:
> On 2019-03-20 16:46:01 [-0700], Paul E. McKenney wrote:
> > Thank you! I reverted v2 and applied this one with the same sort of
> > update. Testing is going well thus far aside from my failing to add
> > the required "=0" after the rcutree.use_softirq. I will probably not
> > be the only one who will run afoul of this, so I updated the commit log
> > and the documentation accordingly, as shown below.
>
> perfect, thank you.
>
> > Thanx, Paul
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index f46b4af96ab9..b807204ffd83 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -629,7 +609,10 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > /* Need to defer quiescent state until everything is enabled. */
> > if (irqs_were_disabled) {
> > /* Enabling irqs does not reschedule, so... */
> > - raise_softirq_irqoff(RCU_SOFTIRQ);
> > + if (!use_softirq)
>
> that exclamation mark needs to go :/

That might explain some of the failures in TREE01, TREE02, TREE03, and
TREE09. TREE01 got a NULL pointer dereference, but in __do_softirq().
So I suspect that this was related. Ditto for TREE02, TREE03, and TREE09.
These also all have CONFIG_PREEMPT=y, and are the only ones run by default
that are set up this way. (Well, so do SRCU-P, TASKS01, and TASKS03, but
they are torturing other forms of RCU.)

Anyway, I applied your fix above and will rerun. The failures happened
within a few seconds in all cases, so a short run should cover this.

Once I get good rcutorture runs, I will ask you to run a heavy-duty run.
Once that passes, I will look at your changes in more detail.

Thanx, Paul

> > + raise_softirq_irqoff(RCU_SOFTIRQ);
> > + else
> > + invoke_rcu_core();
> > } else {
> > /* Enabling BH or preempt does reschedule, so... */
> > set_tsk_need_resched(current);
>
> Sebastian
>

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