[fuse] fix deadlock between fuse_put_super() and request_end()

A deadlock was possible, when the last reference to the superblock was
held due to a background request containing a file reference.

Releasing the file would release the vfsmount which in turn would
release the superblock.  Since sbput_sem is held during the fput() and
fuse_put_super() tries to acquire this same semaphore, a deadlock
results.

The chosen soltuion is to get rid of sbput_sem, and instead use the
spinlock to ensure the referenced inodes/file are released only once.
Since the actual release may sleep, defer these outside the locked
region, but using local variables instead of the structure members.

This is a much more rubust solution.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>

fs/fuse/dev.c

@@ -120,20 +120,14 @@ void fuse_put_request(struct fuse_conn *fc, struct fuse_req *req)
 	}
 }
 
-void fuse_release_background(struct fuse_conn *fc, struct fuse_req *req)
+void fuse_remove_background(struct fuse_conn *fc, struct fuse_req *req)
 {
-	iput(req->inode);
-	iput(req->inode2);
-	if (req->file)
-		fput(req->file);
-	spin_lock(&fc->lock);
-	list_del(&req->bg_entry);
+	list_del_init(&req->bg_entry);
 	if (fc->num_background == FUSE_MAX_BACKGROUND) {
 		fc->blocked = 0;
 		wake_up_all(&fc->blocked_waitq);
 	}
 	fc->num_background--;
-	spin_unlock(&fc->lock);
 }
 
 /*
@@ -163,17 +157,27 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
 		wake_up(&req->waitq);
 		fuse_put_request(fc, req);
 	} else {
+		struct inode *inode = req->inode;
+		struct inode *inode2 = req->inode2;
+		struct file *file = req->file;
 		void (*end) (struct fuse_conn *, struct fuse_req *) = req->end;
 		req->end = NULL;
+		req->inode = NULL;
+		req->inode2 = NULL;
+		req->file = NULL;
+		if (!list_empty(&req->bg_entry))
+			fuse_remove_background(fc, req);
 		spin_unlock(&fc->lock);
-		down_read(&fc->sbput_sem);
-		if (fc->mounted)
-			fuse_release_background(fc, req);
-		up_read(&fc->sbput_sem);
+
 		if (end)
 			end(fc, req);
 		else
 			fuse_put_request(fc, req);
+
+		if (file)
+			fput(file);
+		iput(inode);
+		iput(inode2);
 	}
 }
 

fs/fuse/fuse_i.h

@@ -255,15 +255,9 @@ struct fuse_conn {
 	/** waitq for blocked connection */
 	wait_queue_head_t blocked_waitq;
 
-	/** RW semaphore for exclusion with fuse_put_super() */
-	struct rw_semaphore sbput_sem;
-
 	/** The next unique request id */
 	u64 reqctr;
 
-	/** Mount is active */
-	unsigned mounted;
-
 	/** Connection established, cleared on umount, connection
 	    abort and device release */
 	unsigned connected;
@@ -474,11 +468,11 @@ void request_send_noreply(struct fuse_conn *fc, struct fuse_req *req);
 void request_send_background(struct fuse_conn *fc, struct fuse_req *req);
 
 /**
- * Release inodes and file associated with background request
+ * Remove request from the the background list
  */
-void fuse_release_background(struct fuse_conn *fc, struct fuse_req *req);
+void fuse_remove_background(struct fuse_conn *fc, struct fuse_req *req);
 
-/* Abort all requests */
+/** Abort all requests */
 void fuse_abort_conn(struct fuse_conn *fc);
 
 /**

fs/fuse/inode.c

@@ -204,17 +204,26 @@ static void fuse_put_super(struct super_block *sb)
 {
 	struct fuse_conn *fc = get_fuse_conn_super(sb);
 
-	down_write(&fc->sbput_sem);
-	while (!list_empty(&fc->background))
-		fuse_release_background(fc,
-					list_entry(fc->background.next,
-						   struct fuse_req, bg_entry));
-
 	spin_lock(&fc->lock);
-	fc->mounted = 0;
 	fc->connected = 0;
+	while (!list_empty(&fc->background)) {
+		struct fuse_req *req = list_entry(fc->background.next,
+						  struct fuse_req, bg_entry);
+		struct inode *inode = req->inode;
+		struct inode *inode2 = req->inode2;
+
+		/* File would hold a reference to vfsmount */
+		BUG_ON(req->file);
+		req->inode = NULL;
+		req->inode2 = NULL;
+		fuse_remove_background(fc, req);
+
+		spin_unlock(&fc->lock);
+		iput(inode);
+		iput(inode2);
+		spin_lock(&fc->lock);
+	}
 	spin_unlock(&fc->lock);
-	up_write(&fc->sbput_sem);
 	/* Flush all readers on this fs */
 	kill_fasync(&fc->fasync, SIGIO, POLL_IN);
 	wake_up_all(&fc->waitq);
@@ -386,7 +395,6 @@ static struct fuse_conn *new_conn(void)
 		INIT_LIST_HEAD(&fc->processing);
 		INIT_LIST_HEAD(&fc->io);
 		INIT_LIST_HEAD(&fc->background);
-		init_rwsem(&fc->sbput_sem);
 		kobj_set_kset_s(fc, connections_subsys);
 		kobject_init(&fc->kobj);
 		atomic_set(&fc->num_waiting, 0);
@@ -541,7 +549,6 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 		goto err_free_req;
 
 	sb->s_root = root_dentry;
-	fc->mounted = 1;
 	fc->connected = 1;
 	kobject_get(&fc->kobj);
 	file->private_data = fc;