[PATCH 2/2] workqueue: Fix affinity of kworkers attached during late hotplug

[Valentin Schneider <valentin.schneider@arm.com>]

Per-CPU kworkers forcefully migrated away by hotplug via
workqueue_offline_cpu() can end up spawning more kworkers via

  manage_workers() -> maybe_create_worker()

Workers created at this point will be bound using

  pool->attrs->cpumask

which in this case is wrong, as the hotplug state machine already migrated
all pinned kworkers away from this CPU. This ends up triggering the BUG_ON
condition is sched_cpu_dying() (i.e. there's a kworker enqueued on the
dying rq).

Special-case workers being attached to DISASSOCIATED pools and bind them to
cpu_active_mask, mimicking them being present when workqueue_offline_cpu()
was invoked.

Link: https://lore.kernel.org/r/ff62e3ee994efb3620177bf7b19fab16f4866845.camel@redhat.com
Fixes: 06249738a41a ("workqueue: Manually break affinity on hotplug")
Reported-by: Qian Cai <cai@redhat.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/workqueue.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 9880b6c0e272..fb1418edf85c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1848,19 +1848,29 @@ 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
 	 * definition for details.
+	 *
+	 * Worker might get attached to a pool *after* workqueue_offline_cpu()
+	 * was run - e.g. created by manage_workers() from a kworker which was
+	 * forcefully moved away by hotplug. Kworkers created from this point on
+	 * need to have their affinity changed as if they were present during
+	 * workqueue_offline_cpu().
+	 *
+	 * This will be resolved in rebind_workers().
 	 */
-	if (pool->flags & POOL_DISASSOCIATED)
+	if (pool->flags & POOL_DISASSOCIATED) {
 		worker->flags |= WORKER_UNBOUND;
+		set_cpus_allowed_ptr(worker->task, cpu_active_mask);
+	} else {
+		/*
+		 * 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);
+	}
 
 	list_add_tail(&worker->node, &pool->workers);
 	worker->pool = pool;
-- 
2.27.0