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

Valentin Schneider
2020-12-10T16:40:42Z

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