workqueue: implicit ordered attribute should be overridable

5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be
ordered") automatically enabled ordered attribute for unbound
workqueues w/ max_active == 1.  Because ordered workqueues reject
max_active and some attribute changes, this implicit ordered mode
broke cases where the user creates an unbound workqueue w/ max_active
== 1 and later explicitly changes the related attributes.

This patch distinguishes explicit and implicit ordered setting and
overrides from attribute changes if implict.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered")

2 files changed, 12 insertions, 5 deletions

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index c102ef65cb64..db6dc9dc0482 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -323,6 +323,7 @@ enum {
 
         __WQ_DRAINING           = 1 << 16, /* internal: workqueue is draining */
         __WQ_ORDERED            = 1 << 17, /* internal: workqueue is ordered */
+        __WQ_ORDERED_EXPLICIT   = 1 << 18, /* internal: alloc_ordered_workqueue() */
         __WQ_LEGACY             = 1 << 18, /* internal: create*_workqueue() */
 
         WQ_MAX_ACTIVE           = 512,    /* I like 512, better ideas? */
@@ -422,7 +423,8 @@ __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 | __WQ_ORDERED | (flags), 1, ##args)
+        alloc_workqueue(fmt, WQ_UNBOUND | __WQ_ORDERED |                \
+                        __WQ_ORDERED_EXPLICIT | (flags), 1, ##args)
 
 #define create_workqueue(name)                                          \
         alloc_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, 1, (name))
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index abe4a4971c24..7146ea70a62d 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3744,8 +3744,12 @@ static int apply_workqueue_attrs_locked(struct workqueue_struct *wq,
                 return -EINVAL;
 
         /* creating multiple pwqs breaks ordering guarantee */
-        if (WARN_ON((wq->flags & __WQ_ORDERED) && !list_empty(&wq->pwqs)))
-                return -EINVAL;
+        if (!list_empty(&wq->pwqs)) {
+                if (WARN_ON(wq->flags & __WQ_ORDERED_EXPLICIT))
+                        return -EINVAL;
+
+                wq->flags &= ~__WQ_ORDERED;
+        }
 
         ctx = apply_wqattrs_prepare(wq, attrs);
         if (!ctx)
@@ -4129,13 +4133,14 @@ 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))
+        if (WARN_ON(wq->flags & __WQ_ORDERED_EXPLICIT))
                 return;
 
         max_active = wq_clamp_max_active(max_active, wq->flags, wq->name);
 
         mutex_lock(&wq->mutex);
 
+        wq->flags &= ~__WQ_ORDERED;
         wq->saved_max_active = max_active;
 
         for_each_pwq(pwq, wq)
@@ -5263,7 +5268,7 @@ int workqueue_sysfs_register(struct workqueue_struct *wq)
          * attributes breaks ordering guarantee.  Disallow exposing ordered
          * workqueues.
          */
-        if (WARN_ON(wq->flags & __WQ_ORDERED))
+        if (WARN_ON(wq->flags & __WQ_ORDERED_EXPLICIT))
                 return -EINVAL;
 
         wq->wq_dev = wq_dev = kzalloc(sizeof(*wq_dev), GFP_KERNEL);