Commit d707faa6 authored Nov 13, 2020 by Valentin Schneider Committed by Peter Zijlstra Nov 19, 2020

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: 6d337eab ("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

parent dc824eb8

kernel/sched/core.c

+12 −1

Original line number	Diff line number	Diff line
		@@ -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

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