[PATCH] sched/core: fix affine_move_task failure case

[Daniel Vacek]

Bill Peters reported CPU hangs while offlining/onlining CPUs on s390.

Analyzing the vmcore data shows `stop_one_cpu_nowait()` in `affine_move_task()`
can fail when racing with off-/on-lining resulting in a deadlock waiting for
the pending migration stop work completion which is never done.

Fix this by correctly handling such a condition.

Fixes: 9e81889c7648 ("sched: Fix affine_move_task() self-concurrency")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Bill Peters <wpeters@xxxxxxxxx>
Tested-by: Bill Peters <wpeters@xxxxxxxxx>
Signed-off-by: Daniel Vacek <neelx@xxxxxxxxxx>
---
 kernel/sched/core.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9116bcc903467..d0ff5c611a1c8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3069,8 +3069,17 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 		preempt_disable();
 		task_rq_unlock(rq, p, rf);
 		if (!stop_pending) {
-			stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
-					    &pending->arg, &pending->stop_work);
+			stop_pending =
+				stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
+						    &pending->arg, &pending->stop_work);
+
+			if (!stop_pending) {
+				rq = task_rq_lock(p, rf);
+				pending->stop_pending = false;
+				p->migration_pending = NULL;
+				task_rq_unlock(rq, p, rf);
+				complete_all(&pending->done);
+			}
 		}
 		preempt_enable();
 
-- 
2.43.0