workqueue: wq_pool_mutex protects the attrs-installation

Current wq_pool_mutex doesn't proctect the attrs-installation, it results
that ->unbound_attrs, ->numa_pwq_tbl[] and ->dfl_pwq can only be accessed
under wq->mutex and causes some inconveniences. Example, wq_update_unbound_numa()
has to acquire wq->mutex before fetching the wq->unbound_attrs->no_numa
and the old_pwq.

attrs-installation is a short operation, so this change will no cause any
latency for other operations which also acquire the wq_pool_mutex.

The only unprotected attrs-installation code is in apply_workqueue_attrs(),
so this patch touches code less than comments.

It is also a preparation patch for next several patches which read
wq->unbound_attrs, wq->numa_pwq_tbl[] and wq->dfl_pwq with
only wq_pool_mutex held.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -127,6 +127,11 @@ enum {
  *
  * PR: wq_pool_mutex protected for writes.  Sched-RCU protected for reads.
  *
+ * PW: wq_pool_mutex and wq->mutex protected for writes.  Either for reads.
+ *
+ * PWR: wq_pool_mutex and wq->mutex protected for writes.  Either or
+ *      sched-RCU for reads.
+ *
  * WQ: wq->mutex protected.
  *
  * WR: wq->mutex protected for writes.  Sched-RCU protected for reads.
@@ -247,8 +252,8 @@ struct workqueue_struct {
 	int			nr_drainers;	/* WQ: drain in progress */
 	int			saved_max_active; /* WQ: saved pwq max_active */
 
-	struct workqueue_attrs	*unbound_attrs;	/* WQ: only for unbound wqs */
-	struct pool_workqueue	*dfl_pwq;	/* WQ: only for unbound wqs */
+	struct workqueue_attrs	*unbound_attrs;	/* PW: only for unbound wqs */
+	struct pool_workqueue	*dfl_pwq;	/* PW: only for unbound wqs */
 
 #ifdef CONFIG_SYSFS
 	struct wq_device	*wq_dev;	/* I: for sysfs interface */
@@ -268,7 +273,7 @@ struct workqueue_struct {
 	/* hot fields used during command issue, aligned to cacheline */
 	unsigned int		flags ____cacheline_aligned; /* WQ: WQ_* flags */
 	struct pool_workqueue __percpu *cpu_pwqs; /* I: per-cpu pwqs */
-	struct pool_workqueue __rcu *numa_pwq_tbl[]; /* FR: unbound pwqs indexed by node */
+	struct pool_workqueue __rcu *numa_pwq_tbl[]; /* PWR: unbound pwqs indexed by node */
 };
 
 static struct kmem_cache *pwq_cache;
@@ -349,6 +354,12 @@ static void workqueue_sysfs_unregister(struct workqueue_struct *wq);
 			   lockdep_is_held(&wq->mutex),			\
 			   "sched RCU or wq->mutex should be held")
 
+#define assert_rcu_or_wq_mutex_or_pool_mutex(wq)			\
+	rcu_lockdep_assert(rcu_read_lock_sched_held() ||		\
+			   lockdep_is_held(&wq->mutex) ||		\
+			   lockdep_is_held(&wq_pool_mutex),		\
+			   "sched RCU, wq->mutex or wq_pool_mutex should be held")
+
 #define for_each_cpu_worker_pool(pool, cpu)				\
 	for ((pool) = &per_cpu(cpu_worker_pools, cpu)[0];		\
 	     (pool) < &per_cpu(cpu_worker_pools, cpu)[NR_STD_WORKER_POOLS]; \
@@ -553,7 +564,8 @@ static int worker_pool_assign_id(struct worker_pool *pool)
  * @wq: the target workqueue
  * @node: the node ID
  *
- * This must be called either with pwq_lock held or sched RCU read locked.
+ * This must be called with any of wq_pool_mutex, wq->mutex or sched RCU
+ * read locked.
  * If the pwq needs to be used beyond the locking in effect, the caller is
  * responsible for guaranteeing that the pwq stays online.
  *
@@ -562,7 +574,7 @@ static int worker_pool_assign_id(struct worker_pool *pool)
 static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
 						  int node)
 {
-	assert_rcu_or_wq_mutex(wq);
+	assert_rcu_or_wq_mutex_or_pool_mutex(wq);
 	return rcu_dereference_raw(wq->numa_pwq_tbl[node]);
 }
 
@@ -3479,6 +3491,7 @@ static struct pool_workqueue *numa_pwq_tbl_install(struct workqueue_struct *wq,
 {
 	struct pool_workqueue *old_pwq;
 
+	lockdep_assert_held(&wq_pool_mutex);
 	lockdep_assert_held(&wq->mutex);
 
 	/* link_pwq() can handle duplicate calls */
@@ -3644,10 +3657,9 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 	 * pwqs accordingly.
 	 */
 	get_online_cpus();
-
 	mutex_lock(&wq_pool_mutex);
+
 	ctx = apply_wqattrs_prepare(wq, attrs);
-	mutex_unlock(&wq_pool_mutex);
 
 	/* the ctx has been prepared successfully, let's commit it */
 	if (ctx) {
@@ -3655,6 +3667,7 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 		ret = 0;
 	}
 
+	mutex_unlock(&wq_pool_mutex);
 	put_online_cpus();
 
 	apply_wqattrs_cleanup(ctx);