[dm-devel] [PATCH] a deadlock bug in the kernel-side device mapper code

[malahal at us.ibm.com]

Mike Anderson [andmike at linux.vnet.ibm.com] wrote:
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> > Hi
> > 
> > This is the patch that uses two locks to avoid the deadlock.
> 
> Thanks for doing the patch. 
> 
> I had previously started trying to address this issue using rcu and moving
> dm_copy_name_and_uuid back to being called during dm_build_path_uevent, but
> that patch still had a couple of cases to be addressed.
> 
> In testing your patch without moving where dm_copy_name_and_uuid is called
> I run into a issue during test runs where I receive a BUG_ON for the
> dm_put in dm_copy_name_and_uuid as DMF_FREEING was able to progress (Note:
> this failure case occurs without your path). If the proper dm_get / dm_put
> is added to the dm_uevent functions then there are cases where
> dm_uevent_free becomes the last dm_put resulting in recursion.

Why do we need dm_get() and dm_put() in dm_copy_name_and_uuid()?
dm_copy_name_and_uuid() already has access to md and there shouldn't be
any need to hold a reference. The caller of dm_copy_name_and_uuid()
should have placed a hold. This is just some unnecessary code and should
not cause a BUG_ON though.

Can you send the BUG_ON stack?

dm_get_from_kobject() seems to be a culprit though. It checks
DMF_FREEING without a lock and then calls dm_get. Here is an untested
patch. Made on top of _name_read_lock patch.


Signed-off-by: malahal at us.ibm.com

diff -r ce6e81a22554 -r 1106e5e3dabc drivers/md/dm-ioctl.c
--- a/drivers/md/dm-ioctl.c	Thu Nov 05 21:35:30 2009 -0800
+++ b/drivers/md/dm-ioctl.c	Mon Nov 09 09:32:03 2009 -0800
@@ -1595,7 +1595,6 @@
 	if (!md)
 		return -ENXIO;
 
-	dm_get(md);
 	mutex_lock(&_name_read_lock);
 	hc = dm_get_mdptr(md);
 	if (!hc || hc->md != md) {
@@ -1610,7 +1609,6 @@
 
 out:
 	mutex_unlock(&_name_read_lock);
-	dm_put(md);
 
 	return r;
 }
diff -r ce6e81a22554 -r 1106e5e3dabc drivers/md/dm.c
--- a/drivers/md/dm.c	Thu Nov 05 21:35:30 2009 -0800
+++ b/drivers/md/dm.c	Mon Nov 09 09:32:03 2009 -0800
@@ -2584,11 +2584,18 @@
 	if (&md->kobj != kobj)
 		return NULL;
 
+	spin_lock(&_minor_lock);
 	if (test_bit(DMF_FREEING, &md->flags) ||
-	    test_bit(DMF_DELETING, &md->flags))
-		return NULL;
+	    test_bit(DMF_DELETING, &md->flags)) {
+		md = NULL;
+		goto out;
+	}
 
 	dm_get(md);
+
+out:
+	spin_unlock(&_minor_lock);
+
 	return md;
 }