[dm-devel] dm-mpath: do not change SCSI device handler

[Mikulas Patocka]

On Wed, 3 Apr 2013, Mike Snitzer wrote:

> On Tue, Apr 02 2013 at  8:04pm -0400,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > Hi
> > 
> > This fixes BZ 912245 and 902595.
> > 
> > Mikulas
> > 
> > ---
>
> We should include the currently attached scsi_dh in the DMERR message,
> e.g.:
> 	attached_handler_name = scsi_dh_attached_handler_name(q, GFP_KERNEL);
> 	DMERR("The %s device handler for devices %s is already attached.  ...",
> 	      attached_handler_name, p->path.dev->name);
> 	kfree(attached_handler_name);
> 	goto bad;
> 
> Otherwise, looks good.
> 
> Acked-by: Mike Snitzer <snitzer at redhat.com>
> 

OK. This is the patch with these changes.

---

dm-mpath: do not change SCSI device handler

This patch prevents the multipath target from changing the device handler.
This fixes a kernel crash that can happen when changing the device
handler.

When we reload a multipath device, there are two instances of the
multipath target - the first instance that is active and the second
instance that is being constructed with "ctr" method.

If the multipath constructor finds out that the device is using a
different device handler, it detaches the existing handler and attaches a
new handler. However, the first instance of the multipath target still
exists and processes requests. If the first instance sends some
path-management request with scsi_dh_activate and the second instance
detaches the device handler while the path-management request is in
flight, a crash happens. The reason for the crash is that the endio
routine for the path-management request is working with structures that
were freed when the handler was detached.

There is no practical need to change device handlers on an active device,
so this patch disables it.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

---
 drivers/md/dm-mpath.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Index: linux-3.9-rc5-fast/drivers/md/dm-mpath.c
===================================================================
--- linux-3.9-rc5-fast.orig/drivers/md/dm-mpath.c	2013-04-02 21:54:25.000000000 +0200
+++ linux-3.9-rc5-fast/drivers/md/dm-mpath.c	2013-04-03 22:51:11.000000000 +0200
@@ -618,12 +618,12 @@ static struct pgpath *parse_path(struct 
 		 */
 		r = scsi_dh_attach(q, m->hw_handler_name);
 		if (r == -EBUSY) {
-			/*
-			 * Already attached to different hw_handler:
-			 * try to reattach with correct one.
-			 */
-			scsi_dh_detach(q);
-			r = scsi_dh_attach(q, m->hw_handler_name);
+			const char *a = scsi_dh_attached_handler_name(q, GFP_KERNEL);
+			ti->error = "a different device handler is already attached";
+			DMERR("The %s device handler for device %s is already attached. Can't attach new handler %s",
+				a ? a : "none", p->path.dev->name, m->hw_handler_name);
+			kfree(a);
+			goto bad;
 		}
 
 		if (r < 0) {