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Subject[patch 02/48] nbd: fix I/O hang on disconnected nbds
2.6.28-stable review patch.  If anyone has any objections, please let us know.


From: Paul Clements <>

commit 4d48a542b42747c36a5937447d9c3de7c897ea50 upstream.

Fix a problem that causes I/O to a disconnected (or partially initialized)
nbd device to hang indefinitely. To reproduce:

# ioctl NBD_SET_SIZE_BLOCKS /dev/nbd23 514048
# dd if=/dev/nbd23 of=/dev/null bs=4096 count=1


This can also occur when an nbd device loses its nbd-client/server
connection. Although we clear the queue of any outstanding I/Os after the
client/server connection fails, any additional I/Os that get queued later
will hang.

This bug may also be the problem reported in this bug report:

Testing would need to be performed to determine if the two issues are the

This problem was introduced by the new request handling thread code ("NBD:
allow nbd to be used locally", 3/2008), which entered into mainline around

The fix, which is fairly simple, is to restore the check for lo->sock
being NULL in do_nbd_request. This causes I/O to an uninitialized nbd to
immediately fail with an I/O error, as it did prior to the introduction of
this bug.

Signed-off-by: Paul Clements <>
Reported-by: Jon Nelson <>
Acked-by: Pavel Machek <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

drivers/block/nbd.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -549,6 +549,15 @@ static void do_nbd_request(struct reques

BUG_ON(lo->magic != LO_MAGIC);

+ if (unlikely(!lo->sock)) {
+ printk(KERN_ERR "%s: Attempted send on closed socket\n",
+ lo->disk->disk_name);
+ req->errors++;
+ nbd_end_request(req);
+ spin_lock_irq(q->queue_lock);
+ continue;
+ }
list_add_tail(&req->queuelist, &lo->waiting_queue);

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