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>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 590f4d048ec7..cecd4ffe2c40 100644
--- a/kernel/workqueue.c
+++ b/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);