[PATCH v4 3/5]nbd: make nbd device wait for its users

["Pranay Kr. Srivastava" <pranjas@gmail.com>]

When a timeout occurs or a recv fails, then
instead of abruplty killing nbd block device
wait for its users to finish.

This is more required when filesystem(s) like
ext2 or ext3 don't expect their buffer heads to
disappear while the filesystem is mounted.

Each open of a nbd device is refcounted, while
the userland program [nbd-client] doing the
NBD_DO_IT ioctl would now wait for any other users
of this device before invalidating the nbd device.

A timedout or a disconnected device, if in use, can't
be used until it has been resetted. The reset happens
when all tasks having this bdev open closes this bdev.

Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
---
 drivers/block/nbd.c | 106 ++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 87 insertions(+), 19 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index e362d44..fb56dd2 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -72,6 +72,8 @@ struct nbd_device {
 #endif
 	/* This is specifically for calling sock_shutdown, for now. */
 	struct work_struct ws_shutdown;
+	struct kref users;
+	struct completion user_completion;
 };
 
 #if IS_ENABLED(CONFIG_DEBUG_FS)
@@ -99,6 +101,8 @@ static int max_part;
 static DEFINE_SPINLOCK(nbd_lock);
 
 static void nbd_ws_func_shutdown(struct work_struct *);
+static void nbd_kref_release(struct kref *);
+static int nbd_size_clear(struct nbd_device *, struct block_device *);
 
 static inline struct device *nbd_to_dev(struct nbd_device *nbd)
 {
@@ -145,11 +149,9 @@ static int nbd_size_set(struct nbd_device *nbd, struct block_device *bdev,
 			int blocksize, int nr_blocks)
 {
 	int ret;
-
 	ret = set_blocksize(bdev, blocksize);
 	if (ret)
 		return ret;
-
 	nbd->blksize = blocksize;
 	nbd->bytesize = (loff_t)blocksize * (loff_t)nr_blocks;
 
@@ -197,6 +199,9 @@ static void nbd_xmit_timeout(unsigned long arg)
 {
 	struct nbd_device *nbd = (struct nbd_device *)arg;
 
+	if (nbd->timedout)
+		return;
+
 	if (list_empty(&nbd->queue_head))
 		return;
 
@@ -472,8 +477,6 @@ static int nbd_thread_recv(struct nbd_device *nbd, struct block_device *bdev)
 		nbd_end_request(nbd, req);
 	}
 
-	nbd_size_clear(nbd, bdev);
-
 	device_remove_file(disk_to_dev(nbd->disk), &dev_attr_pid);
 
 	nbd->task_recv = NULL;
@@ -650,12 +653,13 @@ static int nbd_set_socket(struct nbd_device *nbd, struct socket *sock)
 	int ret = 0;
 
 	spin_lock(&nbd->sock_lock);
-	if (nbd->sock)
+
+	if (nbd->sock || nbd->timedout)
 		ret = -EBUSY;
 	else
 		nbd->sock = sock;
-	spin_unlock(&nbd->sock_lock);
 
+	spin_unlock(&nbd->sock_lock);
 	return ret;
 }
 
@@ -670,6 +674,7 @@ static void nbd_reset(struct nbd_device *nbd)
 	nbd->flags = 0;
 	nbd->xmit_timeout = 0;
 	INIT_WORK(&nbd->ws_shutdown, nbd_ws_func_shutdown);
+	init_completion(&nbd->user_completion);
 	queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
 	del_timer_sync(&nbd->timeout_timer);
 }
@@ -704,6 +709,9 @@ static void nbd_dev_dbg_close(struct nbd_device *nbd);
 static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		       unsigned int cmd, unsigned long arg)
 {
+	if (nbd->timedout || nbd->disconnect)
+		return -EBUSY;
+
 	switch (cmd) {
 	case NBD_DISCONNECT: {
 		struct request sreq;
@@ -733,7 +741,6 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		nbd_clear_que(nbd);
 		BUG_ON(!list_empty(&nbd->queue_head));
 		BUG_ON(!list_empty(&nbd->waiting_queue));
-		kill_bdev(bdev);
 		return 0;
 
 	case NBD_SET_SOCK: {
@@ -752,7 +759,6 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 
 	case NBD_SET_BLKSIZE: {
 		loff_t bsize = div_s64(nbd->bytesize, arg);
-
 		return nbd_size_set(nbd, bdev, arg, bsize);
 	}
 
@@ -804,22 +810,29 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		error = nbd_thread_recv(nbd, bdev);
 		nbd_dev_dbg_close(nbd);
 		kthread_stop(thread);
-		sock_shutdown(nbd);
-
-		mutex_lock(&nbd->tx_lock);
-		nbd->task_recv = NULL;
 
-		nbd_clear_que(nbd);
-		kill_bdev(bdev);
-		nbd_bdev_reset(bdev);
+		sock_shutdown(nbd);
 
 		if (nbd->disconnect) /* user requested, ignore socket errors */
 			error = 0;
 		if (nbd->timedout)
 			error = -ETIMEDOUT;
 
-		nbd_reset(nbd);
+		mutex_lock(&nbd->tx_lock);
+		nbd_clear_que(nbd);
+		nbd->disconnect = true; /* To kill bdev*/
+		mutex_unlock(&nbd->tx_lock);
+		cancel_work_sync(&nbd->ws_shutdown);
+		kref_put(&nbd->users, nbd_kref_release);
+		wait_for_completion(&nbd->user_completion);
 
+		mutex_lock(&bdev->bd_mutex);
+		if (!kref_get_unless_zero(&nbd->users))
+			kref_init(&nbd->users);
+		mutex_unlock(&bdev->bd_mutex);
+
+		mutex_lock(&nbd->tx_lock);
+		nbd_reset(nbd);
 		return error;
 	}
 
@@ -857,19 +870,74 @@ static int nbd_ioctl(struct block_device *bdev, fmode_t mode,
 
 	return error;
 }
+static void nbd_kref_release(struct kref *kref_users)
+{
+	struct nbd_device *nbd = container_of(kref_users, struct nbd_device,
+						users
+						);
+	schedule_work(&nbd->ws_shutdown);
+}
+
+static int nbd_open(struct block_device *bdev, fmode_t mode)
+{
+	struct nbd_device *nbd_dev = bdev->bd_disk->private_data;
+
+	if (!kref_get_unless_zero(&nbd_dev->users))
+		kref_init(&nbd_dev->users);
+
+	pr_debug("Opening nbd_dev %s. Active users = %u\n",
+			bdev->bd_disk->disk_name,
+			atomic_read(&nbd_dev->users.refcount)
+		);
+	return 0;
+}
+
+static void nbd_release(struct gendisk *disk, fmode_t mode)
+{
+	struct nbd_device *nbd_dev = disk->private_data;
+
+	kref_put(&nbd_dev->users,  nbd_kref_release);
+
+	pr_debug("Closing nbd_dev %s. Active users = %u\n",
+			disk->disk_name,
+			atomic_read(&nbd_dev->users.refcount)
+		);
+}
 
 static const struct block_device_operations nbd_fops = {
 	.owner =	THIS_MODULE,
 	.ioctl =	nbd_ioctl,
 	.compat_ioctl =	nbd_ioctl,
+	.open =		nbd_open,
+	.release =	nbd_release
 };
 
+
 static void nbd_ws_func_shutdown(struct work_struct *ws_nbd)
 {
 	struct nbd_device *nbd_dev = container_of(ws_nbd, struct nbd_device,
-			ws_shutdown);
-
-	sock_shutdown(nbd_dev);
+							ws_shutdown
+						);
+
+	struct block_device *bdev = bdget(part_devt(
+						dev_to_part(nbd_to_dev(nbd_dev))
+						)
+					);
+	BUG_ON(!bdev);
+	if (nbd_dev->timedout)
+		sock_shutdown(nbd_dev);
+
+	if (nbd_dev->disconnect) {
+		mutex_lock(&nbd_dev->tx_lock);
+		nbd_dev->task_recv = NULL;
+		nbd_clear_que(nbd_dev);
+		kill_bdev(bdev);
+		nbd_bdev_reset(bdev);
+		mutex_unlock(&nbd_dev->tx_lock);
+		nbd_size_clear(nbd_dev, bdev);
+		complete(&nbd_dev->user_completion);
+	}
+	bdput(bdev);
 }
 
 #if IS_ENABLED(CONFIG_DEBUG_FS)
-- 
1.9.1