On Thu, Oct 31, 2019 at 11:25:11PM +0800, Lai Jiangshan wrote:
>
>
> On 2019/10/31 9:52 下午, 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.
And the code further down is not in a fastpath. So, good point, it
should be find to remove the two early exits below.
Thanx, Paul
> 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
> > >
