sched/core: Add missing completion for affine_move_task() waiters

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.

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/8b62fd1ad1b18def27f18e2ee2df3ff5b36d0762.camel@redhat.com

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a6aaf9fb3400bf10d1819e68469102dca549b753..4d1fd4b783ee454999cdb728a397c3ca4e14577e 100644 (file)
--- 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