[PATCH v4 18/18] make nbd device wait for its users in case of timeout

[Pranay Kr. Srivastava]

When a timeout occurs or a recv fails, then
instead of abruplty killing nbd block device
wait for it's 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.

The change is described below:
a) Add a users count to nbd_device structure.
b) Add a bit flag to nbd_device structure of unsigned long.

If the current user count is not 1 then make nbd-client wait
for the in_use bit to be cleared.

Signed-off-by: Pranay Kr. Srivastava <pranjas@xxxxxxxxx>
---
 drivers/block/nbd.c      | 40 ++++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/nbd.h |  1 +
 2 files changed, 41 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 482a3c0..9b024d8 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -59,6 +59,7 @@ struct nbd_device {
 	int xmit_timeout;
 	atomic_t timedout;
 	bool disconnect; /* a disconnect has been requested by user */
+	u32 users;
 
 	struct timer_list timeout_timer;
 	/* protects initialization and shutdown of the socket */
@@ -69,6 +70,7 @@ struct nbd_device {
 #if IS_ENABLED(CONFIG_DEBUG_FS)
 	struct dentry *dbg_dir;
 #endif
+	unsigned long bflags;	/* word size bit flags for use. */
 };
 
 #if IS_ENABLED(CONFIG_DEBUG_FS)
@@ -822,6 +824,15 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		sock_shutdown(nbd);
 		mutex_lock(&nbd->tx_lock);
 		nbd_clear_que(nbd);
+		/*
+		 * Wait for any users currently using
+		 * this block device.
+		 */
+		mutex_unlock(&nbd->tx_lock);
+		pr_info("Waiting for users to release device %s ...\n",
+						bdev->bd_disk->disk_name);
+		wait_on_bit(&nbd->bflags, NBD_BFLAG_INUSE_BIT, TASK_INTERRUPTIBLE);
+		mutex_lock(&nbd->tx_lock);
 		kill_bdev(bdev);
 		nbd_bdev_reset(bdev);
 
@@ -870,10 +881,39 @@ static int nbd_ioctl(struct block_device *bdev, fmode_t mode,
 	return error;
 }
 
+static int nbd_open(struct block_device *bdev, fmode_t mode)
+{
+	struct nbd_device *nbd_dev = bdev->bd_disk->private_data;
+	nbd_dev->users++;
+	pr_debug("Opening nbd_dev %s. Active users = %u\n",
+			bdev->bd_disk->disk_name, nbd_dev->users);
+	if (nbd_dev->users > 1)
+	{
+		set_bit(NBD_BFLAG_INUSE_BIT, &nbd_dev->bflags);
+	}
+	return 0;
+}
+
+static void nbd_release(struct gendisk *disk, fmode_t mode)
+{
+	struct nbd_device *nbd_dev = disk->private_data;
+	nbd_dev->users--;
+	pr_debug("Closing nbd_dev %s. Active users = %u\n",
+			disk->disk_name, nbd_dev->users);
+	if (nbd_dev->users == 1)
+	{
+		clear_bit(NBD_BFLAG_INUSE_BIT, &nbd_dev->bflags);
+		smp_mb();
+		wake_up_bit(&nbd_dev->bflags, NBD_BFLAG_INUSE_BIT);
+	}
+}
+
 static const struct block_device_operations nbd_fops = {
 	.owner =	THIS_MODULE,
 	.ioctl =	nbd_ioctl,
 	.compat_ioctl =	nbd_ioctl,
+	.open = 	nbd_open,
+	.release = 	nbd_release
 };
 
 #if IS_ENABLED(CONFIG_DEBUG_FS)
diff --git a/include/uapi/linux/nbd.h b/include/uapi/linux/nbd.h
index e08e413..8f3d3f0 100644
--- a/include/uapi/linux/nbd.h
+++ b/include/uapi/linux/nbd.h
@@ -44,6 +44,7 @@ enum {
 /* there is a gap here to match userspace */
 #define NBD_FLAG_SEND_TRIM    (1 << 5) /* send trim/discard */
 
+#define NBD_BFLAG_INUSE_BIT	(1) /* bit number for bflags */
 /* userspace doesn't need the nbd_device structure */
 
 /* These are sent over the network in the request/reply magic fields */
-- 
2.6.2