[PATCH 1/1] sched: Use printk_deferred during __schedule()

[Stephen Brennan]

When the rq lock is held and a printk occurs, some console drivers may
trigger a deadlock by calling into scheduler functions (e.g. scheduling
work). Though printk() is avoided in these contexts, there are some WARN
or WARN_ONCE statements which can trigger deadlocks during context
switch operations. These warnings should not trigger deadlocks. Use the
printk_deferred helper to ensure console drivers aren't called until
after the critical section is over.

Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 848eaa0efe0e..bc8afc3020a4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4861,6 +4861,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	perf_event_task_sched_in(prev, current);
 	finish_task(prev);
 	tick_nohz_task_switch();
+	printk_deferred_exit();
 	finish_lock_switch(rq);
 	finish_arch_post_lock_switch();
 	kcov_finish_switch(current);
@@ -6208,6 +6209,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 	 */
 	rq_lock(rq, &rf);
 	smp_mb__after_spinlock();
+	printk_deferred_enter();
 
 	/* Promote REQ to ACT */
 	rq->clock_update_flags <<= 1;
@@ -6298,6 +6300,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 
 		rq_unpin_lock(rq, &rf);
 		__balance_callbacks(rq);
+		printk_deferred_exit();
 		raw_spin_rq_unlock_irq(rq);
 	}
 }
-- 
2.30.2