On Thu, Aug 10, 2017 at 07:33:07AM +1000, Dave Chinner wrote:

Hmmmm, looking closer at xfs_defer_finish(), just holding the buffer
here isn't sufficient. xfs_defer_finish() can roll the transaction a
number of times and holding the buffer is a one-shot deal. Hence the
buffer held buffer will have BLI_HOLD removed on the next commit
and be unlocked by the second commit, whether it be inside
xfs_defer_finish() or the roll that occurs below.

ISTR a previous discussion with Darrick that we needed something
like xfs_defer_join() with buffers instead of inodes to allow them
to be held across a call to xfs_defer_finish()....

We do.  I actually have patches lying around that remove the
xfs_trans_roll and xfs_defer_finish inode arguments, and instead have a
separate helper for rolling over an inode and adding an inode to the
defer list.

I've not added a magic helper for buffers yet, but that would be a
natural fit into that model.