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

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Mar 07 2017 at 12:05pm -0500,
Jens Axboe <axboe@kernel.dk> wrote:

> On 03/07/2017 09:52 AM, Mike Snitzer wrote:
> > On Tue, Mar 07 2017 at  3:49am -0500,
> > Jack Wang <jinpu.wang@profitbricks.com> wrote:
> > 
> >>
> >>
> >> On 06.03.2017 21:18, Jens Axboe wrote:
> >>> On 03/05/2017 09:40 PM, NeilBrown wrote:
> >>>> On Fri, Mar 03 2017, Jack Wang wrote:
> >>>>>
> >>>>> Thanks Neil for pushing the fix.
> >>>>>
> >>>>> We can optimize generic_make_request a little bit:
> >>>>> - assign bio_list struct hold directly instead init and merge
> >>>>> - remove duplicate code
> >>>>>
> >>>>> I think better to squash into your fix.
> >>>>
> >>>> Hi Jack,
> >>>>  I don't object to your changes, but I'd like to see a response from
> >>>>  Jens first.
> >>>>  My preference would be to get the original patch in, then other changes
> >>>>  that build on it, such as this one, can be added.  Until the core
> >>>>  changes lands, any other work is pointless.
> >>>>
> >>>>  Of course if Jens wants a this merged before he'll apply it, I'll
> >>>>  happily do that.
> >>>
> >>> 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
> 
> Can you run this patch with that test, reverting your DM workaround?

Yeap, will do.  Last time Mikulas tried a similar patch it still
deadlocked.  But I'll give it a go (likely tomorrow).