[PATCH 8/8] xfs: serialise unaligned direct IOs

[Alex Elder]

On Tue, 2011-01-04 at 15:48 +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> When two concurrent unaligned, non-overlapping direct IOs are issued
> to the same block, the direct Io layer will race to zero the block.
> The result is that one of the concurrent IOs will overwrite data
> written by the other IO with zeros. This is demonstrated by the
> xfsqa test 240.
> 
> To avoid this problem, serialise all unaligned direct IOs to an
> inode with a big hammer. We need a big hammer approach as we need to
> serialise AIO as well, so we can't just block writes on locks.
> Hence, the big hammer is calling xfs_ioend_wait() while holding out
> other unaligned direct IOs from starting.
> 
> We don't bother trying to serialised aligned vs unaligned IOs as
> they are overlapping IO and the result of concurrent overlapping IOs
> is undefined - the result of either IO is a valid result so we let
> them race. Hence we only penalise unaligned IO, which already has a
> major overhead compared to aligned IO so this isn't a major problem.

Wow, after the rest of this series it gets easy!
Looks good.

Reviewed-by: Alex Elder <aelder at sgi.com>

> Signed-off-by: Dave Chinner <dchinner at redhat.com>
> ---
>  fs/xfs/linux-2.6/xfs_file.c |   28 ++++++++++++++++++++++++----
>  1 files changed, 24 insertions(+), 4 deletions(-)