[PATCH 3/3] Fix locking error caused by e76bd8d9850c2296a7e8e24c9dce9b5e6b55fe2f

[Rusty Russell]

We can't call cpuset_cpus_allowed_locked() with the rq lock held.
However, the rq lock merely protects us from (1) cpu_online_mask changing
and (2) someone else changing p->cpus_allowed.

The first can't happen because we're being called from a cpu hotplug
notifier.  The second doesn't really matter: we are forcing the task off
a CPU it was affine to, so we're not doing very well anyway.

So we remove the rq lock from this path, and all is good.

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

diff --git a/kernel/sched.c b/kernel/sched.c
index 1aa840a..7acf95f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6126,7 +6126,6 @@ static int __migrate_task_irq(struct task_struct *p, int src_cpu, int 
dest_cpu)
  */
 static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 {
-	unsigned long flags;
 	struct rq *rq;
 	int dest_cpu;
 	/* FIXME: Use cpumask_of_node here. */
@@ -6146,10 +6145,8 @@ again:
 
 	/* No more Mr. Nice Guy. */
 	if (dest_cpu >= nr_cpu_ids) {
-		rq = task_rq_lock(p, &flags);
 		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
 		dest_cpu = cpumask_any_and(cpu_online_mask, &p->cpus_allowed);
-		task_rq_unlock(rq, &flags);
 
 		/*
 		 * Don't tell them about moving exiting tasks or

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