pnfs/blocklayout: serialize GETDEVICEINFO calls

The rpc_pipefs code isn't thread safe, leading to occasional use after
frees when running xfstests generic/241 (dbench).

Signed-off-by: Christoph Hellwig <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Cc: [e-mail] # 3.17.x
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/blocklayout/rpc_pipefs.c

···
 
 	dprintk("%s CREATING PIPEFS MESSAGE\n", __func__);
 
+	mutex_lock(&nn->bl_mutex);
 	bl_pipe_msg.bl_wq = &nn->bl_wq;
 
 	b->simple.len += 4;	/* single volume */
 	if (b->simple.len > PAGE_SIZE)
-		return -EIO;
+		goto out_unlock;
 
 	memset(msg, 0, sizeof(*msg));
 	msg->len = sizeof(*bl_msg) + b->simple.len;
 	msg->data = kzalloc(msg->len, gfp_mask);
 	if (!msg->data)
-		goto out;
+		goto out_free_data;
 
 	bl_msg = msg->data;
 	bl_msg->type = BL_DEVICE_MOUNT,
···
 	rc = rpc_queue_upcall(nn->bl_device_pipe, msg);
 	if (rc < 0) {
 		remove_wait_queue(&nn->bl_wq, &wq);
-		goto out;
+		goto out_free_data;
 	}
 
 	set_current_state(TASK_UNINTERRUPTIBLE);
···
 	if (reply->status != BL_DEVICE_REQUEST_PROC) {
 		printk(KERN_WARNING "%s failed to decode device: %d\n",
 			__func__, reply->status);
-		goto out;
+		goto out_free_data;
 	}
 
 	dev = MKDEV(reply->major, reply->minor);
-out:
+out_free_data:
 	kfree(msg->data);
+out_unlock:
+	mutex_unlock(&nn->bl_mutex);
 	return dev;
 }
 
···
 	struct nfs_net *nn = net_generic(net, nfs_net_id);
 	struct dentry *dentry;
 
+	mutex_init(&nn->bl_mutex);
 	init_waitqueue_head(&nn->bl_wq);
 	nn->bl_device_pipe = rpc_mkpipe_data(&bl_upcall_ops, 0);
 	if (IS_ERR(nn->bl_device_pipe))

fs/nfs/netns.h

···
 	struct rpc_pipe *bl_device_pipe;
 	struct bl_dev_msg bl_mount_reply;
 	wait_queue_head_t bl_wq;
+	struct mutex bl_mutex;
 	struct list_head nfs_client_list;
 	struct list_head nfs_volume_list;
 #if IS_ENABLED(CONFIG_NFS_V4)