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Subject[PATCH RT 18/22] sched/core: Avoid __schedule() being called twice in a row
4.14.63-rt41-rc1 stable review patch.
If anyone has any objections, please let me know.

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

From: Daniel Bristot de Oliveira <bristot@redhat.com>

[ 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@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Tommaso Cucinotta <tommaso.cucinotta@sssup.it>
Cc: Romulo da Silva de Oliveira <romulo.deoliveira@ufsc.br>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[bigeasy: rewrite changelog]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
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

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