On Wed, May 26, 2021 at 12:14:20PM +0100, Valentin Schneider wrote:

On 25/05/21 16:14, Will Deacon wrote:

quoted

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5226cc26a095..1702a60d178d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -1869,6 +1869,7 @@ static struct rq *move_queued_task(struct rq *rq, struct rq_flags *rf,
 struct migration_arg {
      struct task_struct		*task;
      int				dest_cpu;
+	const struct cpumask		*dest_mask;
      struct set_affinity_pending	*pending;
 };

@@ -1917,6 +1918,7 @@ static int migration_cpu_stop(void *data)
      struct set_affinity_pending *pending = arg->pending;
      struct task_struct *p = arg->task;
      int dest_cpu = arg->dest_cpu;
+	const struct cpumask *dest_mask = arg->dest_mask;
      struct rq *rq = this_rq();
      bool complete = false;
      struct rq_flags rf;

@@ -1956,12 +1958,8 @@ static int migration_cpu_stop(void *data)
                      complete = true;
              }

-		if (dest_cpu < 0) {
-			if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
-				goto out;
-
-			dest_cpu = cpumask_any_distribute(&p->cpus_mask);
-		}
+		if (dest_mask && (cpumask_test_cpu(task_cpu(p), dest_mask)))
+			goto out;

IIRC the reason we deferred the pick to migration_cpu_stop() was because of
those insane races involving multiple SCA calls the likes of:

  p->cpus_mask = [0, 1]; p on CPU0

  CPUx                           CPUy                   CPU0

  SCA(p, [2])
    __do_set_cpus_allowed();
    queue migration_cpu_stop()
                                 SCA(p, [3])
                                   __do_set_cpus_allowed();
                                                        migration_cpu_stop()

The stopper needs to use the latest cpumask set by the second SCA despite
having an arg->pending set up by the first SCA. Doesn't this break here?

Yes, well spotted. I was so caught up with the hotplug race that I didn't
even consider a straightforward SCA race. Hurumph.

I'm not sure I've paged back in all of the subtleties laying in ambush
here, but what about the below?

I can't break it, but I'm also not very familiar with this code. Please can
you post it as a proper patch so that I drop this from my series?

Thanks,

Will

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