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

Lai Jiangshan
2019-10-31T15:25:20Z

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@linux.alibaba.com>
>> ---
>> 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
>>