[PATCH RFC v2] nbd: fix I/O hang on dead socket and rate-limit console error

[syzbot]

When an NBD device is configured with a timeout of 0, a closed socket can
lead to a permanent I/O hang. The sequence is as follows: a request is sent
and marked in-flight, but then the socket becomes dead (e.g., due to a
connection failure). Since the socket is dead, no reply will ever arrive.
In nbd_xmit_timeout(), if the configured timeout is 0, the code currently
only checks if the socket has been replaced by comparing cookies. If the
cookie still matches, the request timer is unconditionally reset and the
request stays in-flight forever. This causes tasks to hang indefinitely in
TASK_UNINTERRUPTIBLE, triggering the hung task detector:

INFO: task udevd:5915 blocked in I/O wait for more than 143 seconds.
Call Trace:
<TASK>
schedule+0x164/0x2b0
io_schedule+0x7f/0xd0
folio_wait_bit_common+0x836/0xbc0
do_read_cache_folio+0x1ac/0x590
read_part_sector+0xb6/0x2b0
adfspart_check_POWERTEC+0x9a/0x7a0
bdev_disk_changed+0x851/0x17a0
blkdev_get_whole+0x372/0x510
bdev_open+0x324/0xd70
...
</TASK>

Fix this by checking nsock->dead in addition to the cookie check in
nbd_xmit_timeout(). If the socket is dead, the command is requeued.
Requeuing returns the request to the existing fallback, reconnect, or
failure policy. For the reported configuration where no reconnect is
possible, this results in the request being properly failed, which
terminates the hung operation.

A secondary issue was observed where repeated attempts to connect to an
already-in-use NBD device cause console spam because the "nbd%d already in
use" error message in nbd_genl_connect() is not rate-limited. This can
delay console_unlock() and trigger NMI backtraces. This is fixed by
changing the pr_err() to pr_err_ratelimited().

Fixes: 2c272542baee ("nbd: requeue command if the soecket is changed")
Assisted-by: Gemini:gemini-3.1-pro-preview Gemini:gemini-3-flash-preview syzbot
Reported-by: syzbot+82de77...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=82de77d3f217960f087d
Link: https://syzkaller.appspot.com/ai_job?id=40e56b38-627e-4b49-b7ac-d3a21e77dba2
To: "Jens Axboe" <ax...@kernel.dk>
To: "Josef Bacik" <jo...@toxicpanda.com>
To: <linux...@vger.kernel.org>
To: <n...@other.debian.org>
Cc: <linux-...@vger.kernel.org>

---
v2:
- Updated the commit description to precisely detail the sequence leading to the I/O hang.
- Clarified that requeueing returns the request to the existing fallback/reconnect/failure policy.
- Highlighted the console spam as a secondary issue.
- Switched to pr_err_ratelimited() for the "already in use" error in nbd_genl_connect().

v1:
https://lore.kernel.org/all/b5a8f0cc-71aa-4deb...@mail.kernel.org/T/
---
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 8f10762e9..eedb1c870 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -523,7 +523,7 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req)
blk_rq_bytes(req), (req->timeout / HZ) * cmd->retries);

mutex_lock(&nsock->tx_lock);
- if (cmd->cookie != nsock->cookie) {
+ if (cmd->cookie != nsock->cookie || nsock->dead) {
nbd_requeue_cmd(cmd);
mutex_unlock(&nsock->tx_lock);
mutex_unlock(&cmd->lock);
@@ -2172,7 +2172,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
nbd_put(nbd);
if (index == -1)
goto again;
- pr_err("nbd%d already in use\n", index);
+ pr_err_ratelimited("nbd%d already in use\n", index);
return -EBUSY;
}



base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
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