[PATCH AUTOSEL 4.14 02/11] rcu: Suppress smp_processor_id() complaint in synchronize_rcu_expedited_wait()

[Sasha Levin <sashal@kernel.org>]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 2d7f00b2f01301d6e41fd4a28030dab0442265be ]

The normal grace period's RCU CPU stall warnings are invoked from the
scheduling-clock interrupt handler, and can thus invoke smp_processor_id()
with impunity, which allows them to directly invoke dump_cpu_task().
In contrast, the expedited grace period's RCU CPU stall warnings are
invoked from process context, which causes the dump_cpu_task() function's
calls to smp_processor_id() to complain bitterly in debug kernels.

This commit therefore causes synchronize_rcu_expedited_wait() to disable
preemption around its call to dump_cpu_task().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/rcu/tree_exp.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index f90d10c1c3c8d..843399e98bb37 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -498,7 +498,9 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 				mask = leaf_node_cpu_bit(rnp, cpu);
 				if (!(rnp->expmask & mask))
 					continue;
+				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
 				dump_cpu_task(cpu);
+				preempt_enable();
 			}
 		}
 		jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
-- 
2.39.0