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

[Mike Snitzer]

On Tue, Mar 07 2017 at  3:49am -0500,
Jack Wang <jinpu.wang@xxxxxxxxxxxxxxxx> 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

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?