Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM.. and a bonus rt_mutex deadlock report for absolutely free!

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 12 Jul 2012, Mike Galbraith wrote:
> On Thu, 2012-07-12 at 13:43 +0200, Thomas Gleixner wrote: 
> > rawlock points to ...968 and the node_list to ...970.
> > 
> > struct rt_mutex {
> >         raw_spinlock_t          wait_lock;
> >         struct plist_head       wait_list;
> > 
> > The raw_lock pointer of the plist_head is initialized in
> > __rt_mutex_init() so it points to wait_lock. 
> > 
> > Can you check the offset of wait_list vs. the rt_mutex itself?
> > 
> > I wouldn't be surprised if it's exactly 8 bytes. And then this thing
> > looks like a copied lock with stale pointers to hell. Eew.
> 
> crash> struct rt_mutex -o
> struct rt_mutex {
>    [0] raw_spinlock_t wait_lock;
>    [8] struct plist_head wait_list;

Bingo, that makes it more likely that this is caused by copying w/o
initializing the lock and then freeing the original structure.

A quick check for memcpy finds that __btrfs_close_devices() does a
memcpy of btrfs_device structs w/o initializing the lock in the new
copy, but I have no idea whether that's the place we are looking for.

Thanks,

	tglx

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 43baaf0..06c8ced 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -512,6 +512,7 @@ static int __btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 		new_device->writeable = 0;
 		new_device->in_fs_metadata = 0;
 		new_device->can_discard = 0;
+		spin_lock_init(&new_device->io_lock);
 		list_replace_rcu(&device->dev_list, &new_device->dev_list);
 
 		call_rcu(&device->rcu, free_device);