Re: blk: improve order of bio handling in generic_make_request()

[Lars Ellenberg]

On 7 March 2017 at 17:52, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:

> > On 06.03.2017 21:18, Jens Axboe wrote:
> > > I like the change, and thanks for tackling this. It's been a pending
> > > issue for way too long. I do think we should squash Jack's patch
> > > into the original, as it does clean up the code nicely.
> > >
> > > Do we have a proper test case for this, so we can verify that it
> > > does indeed also work in practice?
> > >
> > Hi Jens,
> >
> > I can trigger deadlock with in RAID1 with test below:
> >
> > I create one md with one local loop device and one remote scsi
> > exported by SRP. running fio with mix rw on top of md, force_close
> > session on storage side. mdx_raid1 is wait on free_array in D state,
> > and a lot of fio also in D state in wait_barrier.
> >
> > With the patch from Neil above, I can no longer trigger it anymore.
> >
> > The discussion was in link below:
> > http://www.spinics.net/lists/raid/msg54680.html
>
> In addition to Jack's MD raid test there is a DM snapshot deadlock test,
> albeit unpolished/needy to get running, see:
> https://www.redhat.com/archives/dm-devel/2017-January/msg00064.html
>
> But to actually test block core's ability to handle this, upstream
> commit d67a5f4b5947aba4bfe9a80a2b86079c215ca755 ("dm: flush queued bios
> when process blocks to avoid deadlock") would need to be reverted.
>
> Also, I know Lars had a drbd deadlock too.  Not sure if Jack's MD test
> is sufficient to coverage for drbd.  Lars?
>

As this is just a slightly different implementation, trading some bytes of
stack
for more local, self-contained, "obvious" code changes (good job!),
but follows the same basic idea as my original RFC [*]  (see the
"inspired-by" tag)
I have no doubt it fixes the issues we are able to provoke with DRBD.
[*] https://lkml.org/lkml/2016/7/19/263
(where I also already suggest to fix the device-mapper issues
by losing the in-device-mapper loop,
relying on the loop in generic_make_request())

Cheers,
    Lars