fanotify: convert access_mutex to spinlock

access_mutex is used only to guard operations on access_list.  There's
no need for sleeping within this lock so just make a spinlock out of it.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Eric Paris <eparis@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/fanotify/fanotify_user.c

@@ -134,7 +134,7 @@ static struct fanotify_perm_event_info *dequeue_event(
 {
 	struct fanotify_perm_event_info *event, *return_e = NULL;
 
-	mutex_lock(&group->fanotify_data.access_mutex);
+	spin_lock(&group->fanotify_data.access_lock);
 	list_for_each_entry(event, &group->fanotify_data.access_list,
 			    fae.fse.list) {
 		if (event->fd != fd)
@@ -144,7 +144,7 @@ static struct fanotify_perm_event_info *dequeue_event(
 		return_e = event;
 		break;
 	}
-	mutex_unlock(&group->fanotify_data.access_mutex);
+	spin_unlock(&group->fanotify_data.access_lock);
 
 	pr_debug("%s: found return_re=%p\n", __func__, return_e);
 
@@ -213,10 +213,10 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 
 		pevent = FANOTIFY_PE(event);
 		pevent->fd = fd;
-		mutex_lock(&group->fanotify_data.access_mutex);
+		spin_lock(&group->fanotify_data.access_lock);
 		list_add_tail(&pevent->fae.fse.list,
 			      &group->fanotify_data.access_list);
-		mutex_unlock(&group->fanotify_data.access_mutex);
+		spin_unlock(&group->fanotify_data.access_lock);
 	}
 #endif
 
@@ -346,7 +346,7 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
 	struct fanotify_perm_event_info *event, *next;
 
-	mutex_lock(&group->fanotify_data.access_mutex);
+	spin_lock(&group->fanotify_data.access_lock);
 
 	atomic_inc(&group->fanotify_data.bypass_perm);
 
@@ -358,7 +358,7 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 		list_del_init(&event->fae.fse.list);
 		event->response = FAN_ALLOW;
 	}
-	mutex_unlock(&group->fanotify_data.access_mutex);
+	spin_unlock(&group->fanotify_data.access_lock);
 
 	wake_up(&group->fanotify_data.access_waitq);
 #endif
@@ -700,7 +700,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 
 	group->fanotify_data.f_flags = event_f_flags;
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
-	mutex_init(&group->fanotify_data.access_mutex);
+	spin_lock_init(&group->fanotify_data.access_lock);
 	init_waitqueue_head(&group->fanotify_data.access_waitq);
 	INIT_LIST_HEAD(&group->fanotify_data.access_list);
 	atomic_set(&group->fanotify_data.bypass_perm, 0);

include/linux/fsnotify_backend.h

@@ -178,7 +178,7 @@ struct fsnotify_group {
 		struct fanotify_group_private_data {
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
 			/* allows a group to block waiting for a userspace response */
-			struct mutex access_mutex;
+			spinlock_t access_lock;
 			struct list_head access_list;
 			wait_queue_head_t access_waitq;
 			atomic_t bypass_perm;