Re: Linux 3.0 release - btrfs possible locking deadlock

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Ed Tomlinson's message of 2011-07-22 19:21:00 -0400:
> On Thursday 21 July 2011 22:59:53 Linus Torvalds wrote:
> > So there it is. Gone are the 2.6.<bignum> days, and 3.0 is out.
> > 
> 
> Hi,
> 
> Managed to get this with btrfs rsync(ing) from ext4 to a btrfs fs with three partitions using raid1.
> 
> [16018.211493] device fsid f7186eeb-60df-4b1a-890a-4a1eb42f81fe devid 1 transid 10 /dev/sdd4
> [16018.230643] btrfs: use lzo compression
> [16018.234619] btrfs: enabling disk space caching
> [25949.414011] 
> [25949.414011] =======================================================
> [25949.416549] [ INFO: possible circular locking dependency detected ]
> [25949.423187] 3.0.0-crc+ #348
> [25949.423187] -------------------------------------------------------
> [25949.423187] rsync/20237 is trying to acquire lock:
> [25949.423187]  (btrfs-extent-01){+.+...}, at: [<ffffffffa047ce88>] btrfs_try_spin_lock+0x78/0xb0 [btrfs]
> [25949.423187] 
> [25949.423187] but task is already holding lock:
> [25949.423187]  (&(&eb->lock)->rlock){+.+...}, at: [<ffffffffa047cee2>] btrfs_clear_lock_blocking+0x22/0x30 [btrfs]
> [25949.423187] 
> [25949.423187] which lock already depends on the new lock.
> 
> Kernel is 3.0.0 without any extras.
> 
> Ideas?

Did this actually deadlock?  lockdep has issues with the btrfs
clear_lock_blocking code, and I need to redo the annotations a bit.  The
problem is that we have the same lock class representing unrelated locks from
different trees.

-chris