[dm-devel] v3.15 dm-mpath regression: cable pull test causes I/O hang

Junichi Nomura j-nomura at ce.jp.nec.com
Tue Jul 8 00:55:14 UTC 2014

On 07/07/14 22:40, Bart Van Assche wrote:
> Thanks for looking into this issue. I have attached the requested
> information to this e-mail for a test run with kernel 3.16-rc4 at the
> initiator side.

Thank you, Bart. The information is helpful.

>From "dmsetup table" output, hardware handler is not used
in your setup. So pg_init is not involved.

> # cat /proc/diskstats
>  253       1 dm-1 127 0 1016 1070 0 0 0 0 1 278360 278820

In-flight IO remains.

> # dmsetup info -c
> Name              Maj Min Stat Open Targ Event  UUID                            
> 26466363537333266 253   1 L--w    1    1      0 mpath-26466363537333266 

Typical reason of remainig IO is device staying as suspended.
But the device state is ok here.

> # dmsetup status
> 26466363537333266: 0 256000 multipath 2 1 0 0 1 1 E 0 2 2 8:48 A 0 0 1 8:160 A 0 0 1

Single path group with both paths being active on dm-1.
But the path group is not active.

I suspect what's happening here is nobody clears m->queue_io:
multipath_busy() returns busy when queue_io=1 while clearing queue_io
needs __choose_pgpath(), which won't be called if multipath_busy() is true.

I think if you run 'sg_inq /dev/dm-1' for example in this case,
the device will start working again.
Since ioctl is not affected by multipath_busy(), somehow the problem
was hidden in many cases by udev activities, for example.

Attached patch should fix the problem.
Could you give it a try?

Jun'ichi Nomura, NEC Corporation

pg_ready() checks the current state of the multipath and may return
false even if a new IO is needed to change the state.

OTOH, if multipath_busy() returns busy, a new IO will not be sent
to multipath target and the state change won't happen. That results
in lock up.

The intent of multipath_busy() is to avoid unnecessary cycles of
dequeue + request_fn + requeue if it is known that multipath device
will requeue.

Such situation would be:
  - path group is being activated
  - there is no path and the multipath is setup to requeue if no path

This patch should fix the problem introduced as a part of this commit:
  commit e809917735ebf1b9a56c24e877ce0d320baee2ec
  dm mpath: push back requests instead of queueing

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index ebfa411..d58343e 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -1620,8 +1620,9 @@ static int multipath_busy(struct dm_target *ti)
 	spin_lock_irqsave(&m->lock, flags);
-	/* pg_init in progress, requeue until done */
-	if (!pg_ready(m)) {
+	/* pg_init in progress or no paths available */
+	if (m->pg_init_in_progress ||
+	    (!m->nr_valid_paths && m->queue_if_no_path)) {
 		busy = 1;
 		goto out;

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