dnotify: use fsnotify group lock helpers

Before commit 9542e6a643fc6 ("nfsd: Containerise filecache laundrette")
nfsd would close open files in direct reclaim context. There is no
guarantee that others memory shrinkers don't do the same and no
guarantee that future shrinkers won't do that.

For example, if overlayfs implements inode cache of fscache would
keep open files to cached objects, inode shrinkers could end up closing
open files to underlying fs.

Direct reclaim from dnotify mark allocation context may try to close
open files that have dnotify marks of the same group and hit a deadlock
on mark_mutex.

Set the FSNOTIFY_GROUP_NOFS flag to prevent going into direct reclaim
from allocations under dnotify group lock and use the safe group lock
helpers.

Link: https://lore.kernel.org/r/[e-mail]
Suggested-by: Jan Kara <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/dnotify/dnotify.c

···
 		return;
 	dn_mark = container_of(fsn_mark, struct dnotify_mark, fsn_mark);
 
-	mutex_lock(&dnotify_group->mark_mutex);
+	fsnotify_group_lock(dnotify_group);
 
 	spin_lock(&fsn_mark->lock);
 	prev = &dn_mark->dn;
···
 		free = true;
 	}
 
-	mutex_unlock(&dnotify_group->mark_mutex);
+	fsnotify_group_unlock(dnotify_group);
 
 	if (free)
 		fsnotify_free_mark(fsn_mark);
···
 	new_dn_mark->dn = NULL;
 
 	/* this is needed to prevent the fcntl/close race described below */
-	mutex_lock(&dnotify_group->mark_mutex);
+	fsnotify_group_lock(dnotify_group);
 
 	/* add the new_fsn_mark or find an old one. */
 	fsn_mark = fsnotify_find_mark(&inode->i_fsnotify_marks, dnotify_group);
···
 	} else {
 		error = fsnotify_add_inode_mark_locked(new_fsn_mark, inode, 0);
 		if (error) {
-			mutex_unlock(&dnotify_group->mark_mutex);
+			fsnotify_group_unlock(dnotify_group);
 			goto out_err;
 		}
 		spin_lock(&new_fsn_mark->lock);
···
 
 	if (destroy)
 		fsnotify_detach_mark(fsn_mark);
-	mutex_unlock(&dnotify_group->mark_mutex);
+	fsnotify_group_unlock(dnotify_group);
 	if (destroy)
 		fsnotify_free_mark(fsn_mark);
 	fsnotify_put_mark(fsn_mark);
···
 					  SLAB_PANIC|SLAB_ACCOUNT);
 	dnotify_mark_cache = KMEM_CACHE(dnotify_mark, SLAB_PANIC|SLAB_ACCOUNT);
 
-	dnotify_group = fsnotify_alloc_group(&dnotify_fsnotify_ops, 0);
+	dnotify_group = fsnotify_alloc_group(&dnotify_fsnotify_ops,
+					     FSNOTIFY_GROUP_NOFS);
 	if (IS_ERR(dnotify_group))
 		panic("unable to allocate fsnotify group for dnotify\n");
 	dnotify_sysctl_init();