[PATCH 5.14 061/432] sched: Prevent balance_push() on remote runqueues

Greg Kroah-Hartman
2021-09-16T17:19:32Z

From: Thomas Gleixner <tglx@linutronix.de>

commit 868ad33bfa3bf39960982682ad3a0f8ebda1656e upstream.

sched_setscheduler() and rt_mutex_setprio() invoke the run-queue balance
callback after changing priorities or the scheduling class of a task. The
run-queue for which the callback is invoked can be local or remote.

That's not a problem for the regular rq::push_work which is serialized with
a busy flag in the run-queue struct, but for the balance_push() work which
is only valid to be invoked on the outgoing CPU that's wrong. It not only
triggers the debug warning, but also leaves the per CPU variable push_work
unprotected, which can result in double enqueues on the stop machine list.

Remove the warning and validate that the function is invoked on the
outgoing CPU.

Fixes: ae7927023243 ("sched: Optimize finish_lock_switch()")
Reported-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/87zgt1hdw7.ffs@tglx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/sched/core.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8536,7 +8536,6 @@ static void balance_push(struct rq *rq)
struct task_struct *push_task = rq->curr;

lockdep_assert_rq_held(rq);
- SCHED_WARN_ON(rq->cpu != smp_processor_id());

/*
* Ensure the thing is persistent until balance_push_set(.on = false);
@@ -8544,9 +8543,10 @@ static void balance_push(struct rq *rq)
rq->balance_callback = &balance_push_callback;

/*
- * Only active while going offline.
+ * Only active while going offline and when invoked on the outgoing
+ * CPU.
*/
- if (!cpu_dying(rq->cpu))
+ if (!cpu_dying(rq->cpu) || rq != this_rq())
return;

/*