From: Lai Jiangshan <laijs@linux.alibaba.com>
To: paulmck@kernel.org
Cc: linux-kernel@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
rcu@vger.kernel.org
Subject: Re: [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore()
Date: Thu, 31 Oct 2019 23:25:11 +0800 [thread overview]
Message-ID: <49c778ff-2187-26fb-1477-bdef6eaf298b@linux.alibaba.com> (raw)
In-Reply-To: <20191031135234.GQ20975@paulmck-ThinkPad-P72>
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
>>
next prev parent reply other threads:[~2019-10-31 15:25 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-31 10:07 [PATCH 00/11] rcu: introduce percpu rcu_preempt_depth Lai Jiangshan
2019-10-31 10:07 ` [PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP Lai Jiangshan
2019-10-31 13:43 ` Paul E. McKenney
2019-10-31 18:19 ` Lai Jiangshan
2019-10-31 19:00 ` Paul E. McKenney
2019-10-31 10:07 ` [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt Lai Jiangshan
2019-10-31 13:47 ` Paul E. McKenney
2019-10-31 14:20 ` Lai Jiangshan
2019-10-31 14:31 ` Paul E. McKenney
2019-10-31 15:14 ` Lai Jiangshan
2019-10-31 18:52 ` Paul E. McKenney
2019-11-01 0:19 ` Boqun Feng
2019-11-01 2:29 ` Lai Jiangshan
2019-10-31 10:07 ` [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore() Lai Jiangshan
2019-10-31 13:52 ` Paul E. McKenney
2019-10-31 15:25 ` Lai Jiangshan [this message]
2019-10-31 18:57 ` Paul E. McKenney
2019-10-31 19:02 ` Paul E. McKenney
2019-10-31 10:07 ` [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs() Lai Jiangshan
2019-10-31 14:10 ` Paul E. McKenney
2019-10-31 14:35 ` Lai Jiangshan
2019-10-31 15:07 ` Paul E. McKenney
2019-10-31 18:33 ` Lai Jiangshan
2019-10-31 22:45 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 05/11] rcu: clean all rcu_read_unlock_special after report qs Lai Jiangshan
2019-11-01 11:54 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 06/11] rcu: clear t->rcu_read_unlock_special in one go Lai Jiangshan
2019-11-01 12:10 ` Paul E. McKenney
2019-11-01 16:58 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 07/11] rcu: set special.b.deferred_qs before wake_up() Lai Jiangshan
2019-10-31 10:08 ` [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting Lai Jiangshan
2019-11-01 12:33 ` Paul E. McKenney
2019-11-16 13:04 ` Lai Jiangshan
2019-11-17 21:53 ` Paul E. McKenney
2019-11-18 1:54 ` Lai Jiangshan
2019-11-18 14:57 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 09/11] rcu: wrap usages of rcu_read_lock_nesting Lai Jiangshan
2019-10-31 10:08 ` [PATCH 10/11] rcu: clear the special.b.need_qs in rcu_note_context_switch() Lai Jiangshan
2019-10-31 10:08 ` [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth Lai Jiangshan
2019-11-01 12:58 ` Paul E. McKenney
2019-11-01 13:13 ` Peter Zijlstra
2019-11-01 14:30 ` Paul E. McKenney
2019-11-01 15:32 ` Lai Jiangshan
2019-11-01 16:21 ` Paul E. McKenney
2019-11-01 15:47 ` Lai Jiangshan
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