[PATCH v2] sched/core: handle affine_move_task failure case gracefully

[Daniel Vacek]

CPU hangs were reported 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 gracefully handling such 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 | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f3951e4a55e5b..40a3c9ff74077 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2871,8 +2871,25 @@ 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);
+			/*
+			 * The state resulting in this failure is not expected
+			 * at this point. At least report a WARNING to be able
+			 * to panic and further debug if reproduced.
+			 */
+			if (WARN_ON(!stop_pending)) {
+				/*
+				 * Then try to handle the failure gracefully
+				 * to prevent the deadlock a few lines later.
+				 */
+				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