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Subject[PATCH] sched/core: Add missing completion for affine_move_task() waiters
Date
Qian reported that some fuzzer issuing sched_setaffinity() ends up stuck on
a wait_for_completion(). The problematic pattern seems to be:

affine_move_task()
// task_running() case
stop_one_cpu();
wait_for_completion(&pending->done);

Combined with, on the stopper side:

migration_cpu_stop()
// Task moved between unlocks and scheduling the stopper
task_rq(p) != rq &&
// task_running() case
dest_cpu >= 0

=> no complete_all()

This can happen with both PREEMPT and !PREEMPT, although !PREEMPT should
be more likely to see this given the targeted task has a much bigger window
to block and be woken up elsewhere before the stopper runs.

Make migration_cpu_stop() always look at pending affinity requests; signal
their completion if the stopper hits a rq mismatch but the task is
still within its allowed mask. When Migrate-Disable isn't involved, this
matches the previous set_cpus_allowed_ptr() vs migration_cpu_stop()
behaviour.

Link: https://lore.kernel.org/lkml/8b62fd1ad1b18def27f18e2ee2df3ff5b36d0762.camel@redhat.com
Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Reported-by: Qian Cai <cai@redhat.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
kernel/sched/core.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 02076e6d3792..fad0a8e62aca 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1923,7 +1923,7 @@ static int migration_cpu_stop(void *data)
else
p->wake_cpu = dest_cpu;

- } else if (dest_cpu < 0) {
+ } else if (dest_cpu < 0 || pending) {
/*
* This happens when we get migrated between migrate_enable()'s
* preempt_enable() and scheduling the stopper task. At that
@@ -1933,6 +1933,17 @@ static int migration_cpu_stop(void *data)
* more likely.
*/

+ /*
+ * The task moved before the stopper got to run. We're holding
+ * ->pi_lock, so the allowed mask is stable - if it got
+ * somewhere allowed, we're done.
+ */
+ if (pending && cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {
+ p->migration_pending = NULL;
+ complete = true;
+ goto out;
+ }
+
/*
* When this was migrate_enable() but we no longer have an
* @pending, a concurrent SCA 'fixed' things and we should be
--
2.27.0
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