Backporting the concurrent direct IO write fix to 3.4

[Kerin Millar <kerframil@xxxxxxxxx>]

Hi,

I recently became aware of commit 507630b2 (use shared ilock mode for
direct IO writes by default). I understand that this fixes a regression
which can have a notable impact upon MySQL performance. This is of
considerable interest to myself because my MySQL servers have to contend
with a really tough workload.

The patch applies cleanly to 3.4 stable but is it actually safe to use
it there? Or does it depend on other changes to XFS, such that I'd be
better off holding out for 3.5? Any advice would be greatly appreciated.

Cheers,

--Kerin