mounting btrfs FS on zfs zvol hangs

Hiya,

yes, you'll probably think that is crazy, but after observing better performance with btrfs in some work loads on md RAID5 than btrfs builtin RAID10, I thought I'd try btrfs on zfs (in-kernel, not fuse) zvol (on raidz) just for a laugh.

While this procedure worked for ext4 and xfs, for btrfs, the mount hangs suggesting there might be something wrong with btrfs and/or zfs.

Here's what I'm doing:

zpool create X raidz /dev/sd{a,b,c,d,e,f}
zfs create -V 6T -o refreservation=0 X/Y
mkfs.btrfs /dev/zvol/X/Y
mount /dev/zvol/X/Y /mnt

backtrace for mount:

mount D 0000000000000009 0 2193 1761 0x00000000
ffff880401b4d9a8 0000000000000082 0000000000000001 0000000000000000 ffff880401b4dfd8 ffff880401b4dfd8 ffff880401b4dfd8 0000000000012a40 ffff8802092f0000 ffff88040ef7dc80 ffff880401b4d988 ffff88041fa732c0

Call Trace:
 [<ffffffff81109af0>] ? __lock_page+0x70/0x70
 [<ffffffff81056e9f>] schedule+0x3f/0x60
 [<ffffffff815e85af>] io_schedule+0x8f/0xd0
 [<ffffffff81109afe>] sleep_on_page+0xe/0x20
 [<ffffffff815e8dcf>] __wait_on_bit+0x5f/0x90
 [<ffffffff81109c68>] wait_on_page_bit+0x78/0x80
 [<ffffffff81081d80>] ? autoremove_wake_function+0x40/0x40
 [<ffffffffa041ab7a>] read_extent_buffer_pages+0x3ca/0x430 [btrfs]
 [<ffffffffa03eec40>] ? btrfs_destroy_pinned_extent+0xb0/0xb0 [btrfs]
 [<ffffffffa03f0daa>] btree_read_extent_buffer_pages.isra.62+0x8a/0xc0 [btrfs]
 [<ffffffffa03f2021>] read_tree_block+0x41/0x60 [btrfs]
 [<ffffffffa03f4735>] open_ctree+0xe75/0x1760 [btrfs]
 [<ffffffff812f2f64>] ? snprintf+0x34/0x40
 [<ffffffffa03d1438>] btrfs_fill_super.isra.38+0x78/0x150 [btrfs]
 [<ffffffff811cf55a>] ? disk_name+0xba/0xc0
 [<ffffffff812efbd7>] ? strlcpy+0x47/0x60
 [<ffffffffa03d2806>] btrfs_mount+0x3c6/0x470 [btrfs]
 [<ffffffff8116ae13>] mount_fs+0x43/0x1b0
 [<ffffffff8118565a>] vfs_kern_mount+0x6a/0xc0
 [<ffffffff81186a54>] do_kern_mount+0x54/0x110
 [<ffffffff811884f4>] do_mount+0x1a4/0x260
 [<ffffffff81188990>] sys_mount+0x90/0xe0
 [<ffffffff815f27c2>] system_call_fastpath+0x16/0x1b

3.0.0-13-generic #22-Ubuntu SMP Wed Nov 2 13:27:26 UTC 2011 x86_64

Best regards,
Stephane