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

[tip-bot2 for Valentin Schneider]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     d707faa64d03d26b529cc4aea59dab1b016d4d33
Gitweb:        https://git.kernel.org/tip/d707faa64d03d26b529cc4aea59dab1b016d4d33
Author:        Valentin Schneider <valentin.schneider@xxxxxxx>
AuthorDate:    Fri, 13 Nov 2020 11:24:14 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 19 Nov 2020 11:25:45 +01:00

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@xxxxxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/8b62fd1ad1b18def27f18e2ee2df3ff5b36d0762.camel@xxxxxxxxxx
---
 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 a6aaf9f..4d1fd4b 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
@@ -1934,6 +1934,17 @@ static int migration_cpu_stop(void *data)
 		 */
 
 		/*
+		 * 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
 		 * valid again. Nothing to do.