do not allow two nbd-clients at same time

[Pavel Machek <pavel@suse.cz>]

Two nbd-clients at same time are bad idea, and cause WARN_ON from nbd
in 2.6.28-rc7 from sysfs_add_one. This simply prevents that from
happening.

To reproduce:

 cat /dev/zero | head -c 10000000 > /tmp/delme.fstest.fs
 nbd-server 9100 -l /anyone.can.connect > /tmp/delme.fstest.fs &
 sleep 1
 nbd-client localhost 9100 /dev/nd0 &
 nbd-client localhost 9100 /dev/nd0 &

Signed-off-by: Pavel Machek <pavel@suse.cz>

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 78e22d7..0da741e 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -413,6 +410,7 @@ static int nbd_do_it(struct nbd_device *
 		nbd_end_request(req);
 
 	sysfs_remove_file(&disk_to_dev(lo->disk)->kobj, &pid_attr.attr);
+	lo->pid = 0;
 	return 0;
 }
 
@@ -627,5 +626,7 @@ static int nbd_ioctl(struct block_device
 		return 0;
 	case NBD_DO_IT:
+		if (lo->pid)
+			return -EBUSY;
 		if (!lo->file)
 			return -EINVAL;
 		thread = kthread_create(nbd_thread, lo, lo->disk->disk_name);

-- 
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