rcu: Let dump_cpu_task() be used without preemption disabled

The commit 2d7f00b ("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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>

Author: ryotakakura98

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);
 	}

kernel/sched/core.c

@@ -9726,7 +9726,7 @@ 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();