virtio_fs: introduce virtio_fs_put_locked helper

Introduce a new helper function virtio_fs_put_locked to encapsulate the
common pattern of releasing a virtio_fs reference while holding a lock.
The existing virtio_fs_put helper will be used to release a virtio_fs
reference while not holding a lock.

Also add an assertion in case the lock is not taken when it should.
Reviewed-by: Idan Zach <izach@nvidia.com>
Reviewed-by: Shai Malin <smalin@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Message-Id: <20240825130716.9506-1-mgurtovoy@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

fs/fuse/virtio_fs.c

@@ -201,18 +201,25 @@ static const struct kobj_type virtio_fs_ktype = {
 };
 
 /* Make sure virtiofs_mutex is held */
-static void virtio_fs_put(struct virtio_fs *fs)
+static void virtio_fs_put_locked(struct virtio_fs *fs)
 {
+	lockdep_assert_held(&virtio_fs_mutex);
+
 	kobject_put(&fs->kobj);
 }
 
+static void virtio_fs_put(struct virtio_fs *fs)
+{
+	mutex_lock(&virtio_fs_mutex);
+	virtio_fs_put_locked(fs);
+	mutex_unlock(&virtio_fs_mutex);
+}
+
 static void virtio_fs_fiq_release(struct fuse_iqueue *fiq)
 {
 	struct virtio_fs *vfs = fiq->priv;
 
-	mutex_lock(&virtio_fs_mutex);
 	virtio_fs_put(vfs);
-	mutex_unlock(&virtio_fs_mutex);
 }
 
 static void virtio_fs_drain_queue(struct virtio_fs_vq *fsvq)
@@ -1052,7 +1059,7 @@ static void virtio_fs_remove(struct virtio_device *vdev)
 
 	vdev->priv = NULL;
 	/* Put device reference on virtio_fs object */
-	virtio_fs_put(fs);
+	virtio_fs_put_locked(fs);
 	mutex_unlock(&virtio_fs_mutex);
 }
 
@@ -1596,9 +1603,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
 
 out_err:
 	kfree(fc);
-	mutex_lock(&virtio_fs_mutex);
 	virtio_fs_put(fs);
-	mutex_unlock(&virtio_fs_mutex);
 	return err;
 }