Re: [PATCH] ext4: revert i_data_sum locking cleanups for dioread_nolock

[Eric Whitney <enwlinux@gmail.com>]

* Theodore Ts'o <tytso@mit.edu>:
> On Thu, Feb 18, 2016 at 11:09:56PM +0100, Jan Kara wrote:
> > OK, I had a look into this. So I'm not 100% what has happened but the
> > following looks likely: Current io_end handling can overwrite io_end
> > pointer in the inode in dioread_nolock mode (nothing prevents unlocked DIO
> > to overwrite pointer of locked DIO and then clear it out). I suspect that
> > the change in i_data_sem locking made this race more visible. Attached
> > patch should fix the issue (I don't see failures of generic/300 with it in
> > dioread_nolock mode). Can you consider this instead of a revert Eric sent?
> 
> Thanks!  That does appear to be it. I dropped the revert, confirmed
> that I could still trivially reproduce the failure, applied patch,
> and ran the test 10 times ("kvm-xfstests -C 10 -c dioread_nolock
> generic/300") and it passed with flying colors.
> 
> > I have also a more complete rewrite of io_end handling which makes the code
> > more comprehensible and avoids storing io_end pointer in the inode (thus
> > avoids similar pitfalls in future) but that is a 4.6 matter. I'll submit
> > the rewrite once xfstests runs complete.
> 
> Great, thanks!
> 
> 					- Ted

I ran the same ten test runs (kvm-xfstests -c dioread_nolock generic/300) on
x86_64 and a full test run (kvm-xfstests -g auto) with the patch applied to
4.5-rc4 without regressions relative to my -rc4 baseline results.  Looks
good to me.

Tested-by: Eric Whitney <enwlinux@gmail.com>