[PATCH RT 18/22] sched/core: Avoid __schedule() being called twice in a row

[Steven Rostedt]

4.14.63-rt41-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>

[ Upstream commit 2bb94c48b2ffaabf8c15a51e5cc1b4c541988cab ]

If a worker invokes schedule() then we may have the call chain:
 schedule()
 -> sched_submit_work()
    -> wq_worker_sleeping()
       -> wake_up_worker()
	  -> wake_up_process().

The last wakeup may cause a schedule which is unnecessary because we are
already in schedule() and do it anyway.

Add a preempt_disable() + preempt_enable_no_resched() around
wq_worker_sleeping() so the context switch could be delayed until
__schedule().

Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Clark Williams <williams@xxxxxxxxxx>
Cc: Tommaso Cucinotta <tommaso.cucinotta@xxxxxxxx>
Cc: Romulo da Silva de Oliveira <romulo.deoliveira@xxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
[bigeasy: rewrite changelog]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/sched/core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fa5b76255f8c..a5ce37b90fca 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3482,10 +3482,15 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	/*
 	 * If a worker went to sleep, notify and ask workqueue whether
 	 * it wants to wake up a task to maintain concurrency.
+	 * As this function is called inside the schedule() context,
+	 * we disable preemption to avoid it calling schedule() again
+	 * in the possible wakeup of a kworker.
 	 */
-	if (tsk->flags & PF_WQ_WORKER)
+	if (tsk->flags & PF_WQ_WORKER) {
+		preempt_disable();
 		wq_worker_sleeping(tsk);
-
+		preempt_enable_no_resched();
+	}
 
 	if (tsk_is_pi_blocked(tsk))
 		return;
-- 
2.18.0