workqueue: reject adjusting max_active or applying attrs to ordered workqueues

Adjusting max_active of or applying new workqueue_attrs to an ordered
workqueue breaks its ordering guarantee.  The former is obvious.  The
latter is because applying attrs creates a new pwq (pool_workqueue)
and there is no ordering constraint between the old and new pwqs.

Make apply_workqueue_attrs() and workqueue_set_max_active() trigger
WARN_ON() if those operations are requested on an ordered workqueue
and fail / ignore respectively.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

include/linux/workqueue.h

@@ -295,6 +295,7 @@ enum {
 	WQ_CPU_INTENSIVE	= 1 << 5, /* cpu instensive workqueue */
 
 	__WQ_DRAINING		= 1 << 16, /* internal: workqueue is draining */
+	__WQ_ORDERED		= 1 << 17, /* internal: workqueue is ordered */
 
 	WQ_MAX_ACTIVE		= 512,	  /* I like 512, better ideas? */
 	WQ_MAX_UNBOUND_PER_CPU	= 4,	  /* 4 * #cpus for unbound wq */
@@ -397,7 +398,7 @@ __alloc_workqueue_key(const char *fmt, unsigned int flags, int max_active,
  * Pointer to the allocated workqueue on success, %NULL on failure.
  */
 #define alloc_ordered_workqueue(fmt, flags, args...)			\
-	alloc_workqueue(fmt, WQ_UNBOUND | (flags), 1, ##args)
+	alloc_workqueue(fmt, WQ_UNBOUND | __WQ_ORDERED | (flags), 1, ##args)
 
 #define create_workqueue(name)						\
 	alloc_workqueue((name), WQ_MEM_RECLAIM, 1)

kernel/workqueue.c

@@ -3494,9 +3494,14 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 	struct pool_workqueue *pwq, *last_pwq;
 	struct worker_pool *pool;
 
+	/* only unbound workqueues can change attributes */
 	if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
 		return -EINVAL;
 
+	/* creating multiple pwqs breaks ordering guarantee */
+	if (WARN_ON((wq->flags & __WQ_ORDERED) && !list_empty(&wq->pwqs)))
+		return -EINVAL;
+
 	pwq = kmem_cache_zalloc(pwq_cache, GFP_KERNEL);
 	if (!pwq)
 		return -ENOMEM;
@@ -3752,6 +3757,10 @@ void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)
 {
 	struct pool_workqueue *pwq;
 
+	/* disallow meddling with max_active for ordered workqueues */
+	if (WARN_ON(wq->flags & __WQ_ORDERED))
+		return;
+
 	max_active = wq_clamp_max_active(max_active, wq->flags, wq->name);
 
 	spin_lock_irq(&workqueue_lock);