workqueue: Restrict affinity change to rescuer

stable inclusion
from stable-5.10.14
commit b80df6517d350df10449a8fff755f24e847214db
bugzilla: 48051

--------------------------------

[ Upstream commit 640f17c8 ]

create_worker() will already set the right affinity using
kthread_bind_mask(), this means only the rescuer will need to change
it's affinity.

Howveer, while in cpu-hot-unplug a regular task is not allowed to run
on online&&!active as it would be pushed away quite agressively. We
need KTHREAD_IS_PER_CPU to survive in that environment.

Therefore set the affinity after getting that magic flag.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NValentin Schneider <valentin.schneider@arm.com>
Tested-by: NValentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210121103506.826629830@infradead.orgSigned-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

kernel/workqueue.c

@@ -1845,12 +1845,6 @@ static void worker_attach_to_pool(struct worker *worker,
 {
 	mutex_lock(&wq_pool_attach_mutex);
 
-	/*
-	 * set_cpus_allowed_ptr() will fail if the cpumask doesn't have any
-	 * online CPUs.  It'll be re-applied when any of the CPUs come up.
-	 */
-	set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
-
 	/*
 	 * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
 	 * stable across this function.  See the comments above the flag
@@ -1859,6 +1853,9 @@ static void worker_attach_to_pool(struct worker *worker,
 	if (pool->flags & POOL_DISASSOCIATED)
 		worker->flags |= WORKER_UNBOUND;
 
+	if (worker->rescue_wq)
+		set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+
 	list_add_tail(&worker->node, &pool->workers);
 	worker->pool = pool;