fuse: add refcount to fuse_dev

This will make it possible to grab the fuse_dev and subsequently release
the file that it came from.

In the above case, fud->fc will be set to FUSE_DEV_FC_DISCONNECTED to
indicate that this is no longer a functional device.

When trying to assign an fc to such a disconnected fuse_dev, the fc is set
to the disconnected state.

Use atomic operations xchg() and cmpxchg() to prevent races.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: Miklos Szeredi

fs/fuse/cuse.c

@@ -527,7 +527,7 @@ static int cuse_channel_open(struct inode *inode, struct file *file)
 	cc->fc.initialized = 1;
 	rc = cuse_send_init(cc);
 	if (rc) {
-		fuse_dev_free(fud);
+		fuse_dev_put(fud);
 		return rc;
 	}
 	file->private_data = fud;

fs/fuse/dev.c

@@ -2540,7 +2540,8 @@ void fuse_wait_aborted(struct fuse_conn *fc)
 int fuse_dev_release(struct inode *inode, struct file *file)
 {
 	struct fuse_dev *fud = fuse_file_to_fud(file);
-	struct fuse_conn *fc = fuse_dev_fc_get(fud);
+	/* Pairs with cmpxchg() in fuse_dev_install() */
+	struct fuse_conn *fc = xchg(&fud->fc, FUSE_DEV_FC_DISCONNECTED);
 
 	if (fc) {
 		struct fuse_pqueue *fpq = &fud->pq;
@@ -2560,8 +2561,12 @@ int fuse_dev_release(struct inode *inode, struct file *file)
 			WARN_ON(fc->iq.fasync != NULL);
 			fuse_abort_conn(fc);
 		}
+		spin_lock(&fc->lock);
+		list_del(&fud->entry);
+		spin_unlock(&fc->lock);
+		fuse_conn_put(fc);
 	}
-	fuse_dev_free(fud);
+	fuse_dev_put(fud);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(fuse_dev_release);

fs/fuse/fuse_dev_i.h

@@ -39,22 +39,23 @@ struct fuse_copy_state {
 	} ring;
 };
 
+/* fud->fc gets assigned to this value when /dev/fuse is closed */
+#define FUSE_DEV_FC_DISCONNECTED ((struct fuse_conn *) 1)
+
 /*
  * Lockless access is OK, because fud->fc is set once during mount and is valid
  * until the file is released.
+ *
+ * fud->fc is set to FUSE_DEV_FC_DISCONNECTED only after the containing file is
+ * released, so result is safe to dereference in most cases.  Exceptions are:
+ * fuse_dev_put() and fuse_fill_super_common().
  */
 static inline struct fuse_conn *fuse_dev_fc_get(struct fuse_dev *fud)
 {
-	/* Pairs with smp_store_release() in fuse_dev_fc_set() */
+	/* Pairs with xchg() in fuse_dev_install() */
 	return smp_load_acquire(&fud->fc);
 }
 
-static inline void fuse_dev_fc_set(struct fuse_dev *fud, struct fuse_conn *fc)
-{
-	/* Pairs with smp_load_acquire() in fuse_dev_fc_get() */
-	smp_store_release(&fud->fc, fc);
-}
-
 static inline struct fuse_dev *fuse_file_to_fud(struct file *file)
 {
 	return file->private_data;

fs/fuse/fuse_i.h

@@ -577,6 +577,9 @@ struct fuse_pqueue {
  * Fuse device instance
  */
 struct fuse_dev {
+	/** Reference count of this object */
+	refcount_t ref;
+
 	/** Issue FUSE_INIT synchronously */
 	bool sync_init;
 
@@ -1344,7 +1347,7 @@ void fuse_conn_put(struct fuse_conn *fc);
 struct fuse_dev *fuse_dev_alloc_install(struct fuse_conn *fc);
 struct fuse_dev *fuse_dev_alloc(void);
 void fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc);
-void fuse_dev_free(struct fuse_dev *fud);
+void fuse_dev_put(struct fuse_dev *fud);
 int fuse_send_init(struct fuse_mount *fm);
 
 /**

fs/fuse/inode.c

@@ -1626,6 +1626,7 @@ struct fuse_dev *fuse_dev_alloc(void)
 	if (!fud)
 		return NULL;
 
+	refcount_set(&fud->ref, 1);
 	pq = kzalloc_objs(struct list_head, FUSE_PQ_HASH_SIZE);
 	if (!pq) {
 		kfree(fud);
@@ -1641,9 +1642,26 @@ EXPORT_SYMBOL_GPL(fuse_dev_alloc);
 
 void fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc)
 {
-	fuse_dev_fc_set(fud, fuse_conn_get(fc));
+	struct fuse_conn *old_fc;
+
 	spin_lock(&fc->lock);
-	list_add_tail(&fud->entry, &fc->devices);
+	/*
+	 * Pairs with:
+	 *  - xchg() in fuse_dev_release()
+	 *  - smp_load_acquire() in fuse_dev_fc_get()
+	 */
+	old_fc = cmpxchg(&fud->fc, NULL, fc);
+	if (old_fc) {
+		/*
+		 * failed to set fud->fc because
+		 *  - it was already set to a different fc
+		 *  - it was set to disconneted
+		 */
+		fc->connected = 0;
+	} else {
+		list_add_tail(&fud->entry, &fc->devices);
+		fuse_conn_get(fc);
+	}
 	spin_unlock(&fc->lock);
 }
 EXPORT_SYMBOL_GPL(fuse_dev_install);
@@ -1661,11 +1679,16 @@ struct fuse_dev *fuse_dev_alloc_install(struct fuse_conn *fc)
 }
 EXPORT_SYMBOL_GPL(fuse_dev_alloc_install);
 
-void fuse_dev_free(struct fuse_dev *fud)
+void fuse_dev_put(struct fuse_dev *fud)
 {
-	struct fuse_conn *fc = fuse_dev_fc_get(fud);
+	struct fuse_conn *fc;
+
+	if (!refcount_dec_and_test(&fud->ref))
+		return;
 
-	if (fc) {
+	fc = fuse_dev_fc_get(fud);
+	if (fc && fc != FUSE_DEV_FC_DISCONNECTED) {
+		/* This is the virtiofs case (fuse_dev_release() not called) */
 		spin_lock(&fc->lock);
 		list_del(&fud->entry);
 		spin_unlock(&fc->lock);
@@ -1675,7 +1698,7 @@ void fuse_dev_free(struct fuse_dev *fud)
 	kfree(fud->pq.processing);
 	kfree(fud);
 }
-EXPORT_SYMBOL_GPL(fuse_dev_free);
+EXPORT_SYMBOL_GPL(fuse_dev_put);
 
 static void fuse_fill_attr_from_inode(struct fuse_attr *attr,
 				      const struct fuse_inode *fi)

fs/fuse/virtio_fs.c

@@ -486,7 +486,7 @@ static void virtio_fs_free_devs(struct virtio_fs *fs)
 		if (!fsvq->fud)
 			continue;
 
-		fuse_dev_free(fsvq->fud);
+		fuse_dev_put(fsvq->fud);
 		fsvq->fud = NULL;
 	}
 }