Re: [PATCH] rcu: Let rcu_dump_task() be used without preemption disabled

[takakura]

Hi Paul,

On Mon, 8 July 2024, Paul E. McKenney wrote:
>On Fri, Jun 28, 2024 at 01:18:26PM +0900, takakura@xxxxxxxxxxxxx wrote:
>> From: Ryo Takakura <takakura@xxxxxxxxxxxxx>
>> 
>> The commit 2d7f00b2f0130 ("rcu: Suppress smp_processor_id() complaint
>> in synchronize_rcu_expedited_wait()") disabled preemption around
>> dump_cpu_task() to suppress warning on its usage within preemtible context.
>> 
>> Calling dump_cpu_task() doesn't required to be in non-preemptible context
>> except for suppressing the smp_processor_id() warning.
>> As the smp_processor_id() is evaluated along with in_hardirq()
>> to check if it's in interrupt context, this patch removes the need
>> for its preemtion disablement by reordering the condition so that
>> smp_processor_id() only gets evaluated when it's in interrupt context.
>> 
>> Signed-off-by: Ryo Takakura <takakura@xxxxxxxxxxxxx>
>
>Hearing no objections, I pulled this in for further review and testing.
>
>I had to hand-apply this due to a recent conflicting change in the
>-rcu tree, so could you please check the version below in case I messed
>something up?
>
>							Thanx, Paul

Thanks for preparing the patch!
I checked it on the rcu tree and looks good to me.

If possible, could you replace the title with s/rcu_dump_task()/dump_cpu_task()/ 
when applying?
I made a mistake with the title where dump_cpu_task() is the one being modified, 
not rcu_dump_task(). I'm sorry for the confusion.

Sincerely,
Ryo Takakura

>
>------------------------------------------------------------------------
>
>commit ad6647a70f239aa9f2741b2f5a828a4483122a26
>Author: Ryo Takakura <takakura@xxxxxxxxxxxxx>
>Date:   Fri Jun 28 13:18:26 2024 +0900
>
>    rcu: Let rcu_dump_task() be used without preemption disabled
>    
>    The commit 2d7f00b2f0130 ("rcu: Suppress smp_processor_id() complaint
>    in synchronize_rcu_expedited_wait()") disabled preemption around
>    dump_cpu_task() to suppress warning on its usage within preemtible context.
>    
>    Calling dump_cpu_task() doesn't required to be in non-preemptible context
>    except for suppressing the smp_processor_id() warning.
>    As the smp_processor_id() is evaluated along with in_hardirq()
>    to check if it's in interrupt context, this patch removes the need
>    for its preemtion disablement by reordering the condition so that
>    smp_processor_id() only gets evaluated when it's in interrupt context.
>    
>    Signed-off-by: Ryo Takakura <takakura@xxxxxxxxxxxxx>
>    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
>diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
>index d4be644afb50..c5d9a7eb0803 100644
>--- a/kernel/rcu/tree_exp.h
>+++ b/kernel/rcu/tree_exp.h
>@@ -597,9 +597,7 @@ static void synchronize_rcu_expedited_stall(unsigned long jiffies_start, unsigne
> 			mask = leaf_node_cpu_bit(rnp, cpu);
> 			if (!(READ_ONCE(rnp->expmask) & mask))
> 				continue;
>-			preempt_disable(); // For smp_processor_id() in dump_cpu_task().
> 			dump_cpu_task(cpu);
>-			preempt_enable();
> 		}
> 		rcu_exp_print_detail_task_stall_rnp(rnp);
> 	}
>diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>index 05afa2932b5e..bdb0e0328f6a 100644
>--- a/kernel/sched/core.c
>+++ b/kernel/sched/core.c
>@@ -11485,7 +11485,7 @@ struct cgroup_subsys cpu_cgrp_subsys = {
> 
> void dump_cpu_task(int cpu)
> {
>-	if (cpu == smp_processor_id() && in_hardirq()) {
>+	if (in_hardirq() && cpu == smp_processor_id()) {
> 		struct pt_regs *regs;
> 
> 		regs = get_irq_regs();