workqueue: require CPU hotplug read exclusion for apply_workqueue_attrs

Change the calling convention for apply_workqueue_attrs to require CPU
hotplug read exclusion.

Avoids lockdep complaints about nested calls to get_online_cpus in a
future patch where padata calls apply_workqueue_attrs when changing
other CPU-hotplug-sensitive data structures with the CPU read lock
already held.
Signed-off-by: NDaniel Jordan <daniel.m.jordan@oracle.com>
Acked-by: NTejun Heo <tj@kernel.org>
Acked-by: NSteffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-crypto@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

kernel/workqueue.c

@@ -4030,6 +4030,8 @@ static int apply_workqueue_attrs_locked(struct workqueue_struct *wq,
  *
  * Performs GFP_KERNEL allocations.
  *
+ * Assumes caller has CPU hotplug read exclusion, i.e. get_online_cpus().
+ *
  * Return: 0 on success and -errno on failure.
  */
 int apply_workqueue_attrs(struct workqueue_struct *wq,
@@ -4037,9 +4039,11 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 {
 	int ret;
 
-	apply_wqattrs_lock();
+	lockdep_assert_cpus_held();
+
+	mutex_lock(&wq_pool_mutex);
 	ret = apply_workqueue_attrs_locked(wq, attrs);
-	apply_wqattrs_unlock();
+	mutex_unlock(&wq_pool_mutex);
 
 	return ret;
 }
@@ -4152,16 +4156,21 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 			mutex_unlock(&wq->mutex);
 		}
 		return 0;
-	} else if (wq->flags & __WQ_ORDERED) {
+	}
+
+	get_online_cpus();
+	if (wq->flags & __WQ_ORDERED) {
 		ret = apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]);
 		/* there should only be single pwq for ordering guarantee */
 		WARN(!ret && (wq->pwqs.next != &wq->dfl_pwq->pwqs_node ||
 			      wq->pwqs.prev != &wq->dfl_pwq->pwqs_node),
 		     "ordering guarantee broken for workqueue %s\n", wq->name);
-		return ret;
 	} else {
-		return apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
+		ret = apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
 	}
+	put_online_cpus();
+
+	return ret;
 }
 
 static int wq_clamp_max_active(int max_active, unsigned int flags,