Re: [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore()

[Lai Jiangshan]

On 2019/10/31 9:52 äå, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:07:58AM +0000, Lai Jiangshan wrote:
> > Remove several unneeded return.
> >
> > It doesn't need to return earlier after every code block.
> > The code protects itself and be safe to fall through because
> > every code block has its own condition tests.
> >
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >   kernel/rcu/tree_plugin.h | 14 +-------------
> >   1 file changed, 1 insertion(+), 13 deletions(-)
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 59ef10da1e39..82595db04eec 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -439,19 +439,10 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> >   	 * t->rcu_read_unlock_special cannot change.
> >   	 */
> >   	special = t->rcu_read_unlock_special;
> > -	rdp = this_cpu_ptr(&rcu_data);
> > -	if (!special.s && !rdp->exp_deferred_qs) {
> > -		local_irq_restore(flags);
> > -		return;
> > -	}
>
> The point of this check is the common case of this function being invoked
> when both fields are zero, avoiding the below redundant store and all the
> extra checks of subfields of special.
>
> Or are you saying that current compilers figure all this out?

No.

So, I have to keep the first/above return branch.

Any reasons to keep the following 2 return branches?
There is no redundant store and the load for the checks
are hot in the cache if the condition for return is met.

Thanks.
Lai

> 							Thanx, Paul
>
> >   	t->rcu_read_unlock_special.b.deferred_qs = false;
> >   	if (special.b.need_qs) {
> >   		rcu_qs();
> >   		t->rcu_read_unlock_special.b.need_qs = false;
> > -		if (!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) {
> > -			local_irq_restore(flags);
> > -			return;
> > -		}
> >   	}
> >   
> >   	/*
> > @@ -460,12 +451,9 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> >   	 * tasks are handled when removing the task from the
> >   	 * blocked-tasks list below.
> >   	 */
> > +	rdp = this_cpu_ptr(&rcu_data);
> >   	if (rdp->exp_deferred_qs) {
> >   		rcu_report_exp_rdp(rdp);
> > -		if (!t->rcu_read_unlock_special.s) {
> > -			local_irq_restore(flags);
> > -			return;
> > -		}
> >   	}
> >   
> >   	/* Clean up if blocked during RCU read-side critical section. */
> > --
> > 2.20.1