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

[takakura]

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>
---
 kernel/rcu/tree_exp.h | 2 --
 kernel/sched/core.c   | 2 +-
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 8a1d9c8bd9f7..80c461b26144 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -617,9 +617,7 @@ static void synchronize_rcu_expedited_wait(void)
 				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 bcf2c4cc0522..38efbdafe3d4 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();
-- 
2.34.1