cgroup: reduce cgroup_file_kn_lock hold time in cgroup_file_notify()

cgroup_file_notify() calls kernfs_notify() while holding the global
cgroup_file_kn_lock.  kernfs_notify() does non-trivial work including
wake_up_interruptible() and acquisition of a second global spinlock
(kernfs_notify_lock), inflating the hold time.

Take a kernfs_get() reference under the lock and call kernfs_notify()
after dropping it, following the pattern from cgroup_file_show().

Reported-by: Jakub Kicinski <[email protected]>
Signed-off-by: Shakeel Butt <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: shakeelb

kernel/cgroup/cgroup.c

@@ -4632,6 +4632,7 @@ int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 void cgroup_file_notify(struct cgroup_file *cfile)
 {
 	unsigned long flags;
+	struct kernfs_node *kn = NULL;
 
 	spin_lock_irqsave(&cgroup_file_kn_lock, flags);
 	if (cfile->kn) {
@@ -4641,11 +4642,17 @@ void cgroup_file_notify(struct cgroup_file *cfile)
 		if (time_in_range(jiffies, last, next)) {
 			timer_reduce(&cfile->notify_timer, next);
 		} else {
-			kernfs_notify(cfile->kn);
+			kn = cfile->kn;
+			kernfs_get(kn);
 			cfile->notified_at = jiffies;
 		}
 	}
 	spin_unlock_irqrestore(&cgroup_file_kn_lock, flags);
+
+	if (kn) {
+		kernfs_notify(kn);
+		kernfs_put(kn);
+	}
 }
 EXPORT_SYMBOL_GPL(cgroup_file_notify);