Re: [PATCH v2] block: flush queued bios when the process blocks

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Oct 08 2015 at 11:08am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Thu, Oct 08 2015 at 11:04am -0400,
> Mikulas Patocka <mpatocka@redhat.com> wrote:
> 
> > 
> > 
> > On Tue, 6 Oct 2015, Mike Snitzer wrote:
> > 
> > > To give others context for why I'm caring about this issue again, this
> > > recent BZ against 4.3-rc served as a reminder that we _need_ a fix:
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1267650
> > > 
> > > FYI, I cleaned up the plug-based approach a bit further, here is the
> > > incremental patch:
> > > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=f73d001ec692125308accbb5ca26f892f949c1b6
> > > 
> > > And here is a new version of the overall combined patch (sharing now
> > > before I transition to looking at alternatives, though my gut is the use
> > > of a plug in generic_make_request really wouldn't hurt us.. famous last
> > > words):
> > > 
> > >  block/bio.c            | 82 +++++++++++++-------------------------------------
> > >  block/blk-core.c       | 21 ++++++++-----
> > >  drivers/md/dm-bufio.c  |  2 +-
> > >  drivers/md/raid1.c     |  6 ++--
> > >  drivers/md/raid10.c    |  6 ++--
> > >  include/linux/blkdev.h | 11 +++++--
> > >  include/linux/sched.h  |  4 ---
> > >  7 files changed, 51 insertions(+), 81 deletions(-)
> > > 
> ...
> > > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> > > index 2dd3308..c2bff16 100644
> > > --- a/drivers/md/dm-bufio.c
> > > +++ b/drivers/md/dm-bufio.c
> > > @@ -168,7 +168,7 @@ static inline int dm_bufio_cache_index(struct dm_bufio_client *c)
> > >  #define DM_BUFIO_CACHE(c)	(dm_bufio_caches[dm_bufio_cache_index(c)])
> > >  #define DM_BUFIO_CACHE_NAME(c)	(dm_bufio_cache_names[dm_bufio_cache_index(c)])
> > >  
> > > -#define dm_bufio_in_request()	(!!current->bio_list)
> > > +#define dm_bufio_in_request()	(current->plug && !!current->plug->bio_list)
> > 
> > This condition is repeated several times throughout the whole patch - so 
> > maybe you should make it a function in block device header file.
> 
> Yeah, I thought of that too but forgot to come back to it.  Will do,
> thanks.
> 
> FYI, I found another bug in my last patch and fixed it up.  I'll get
> some refactoring done (including your suggestion), actually _test_ the
> code (e.g. verify all of lvm testsuite passes) and then send out v3.

Turns out that this change:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=2639638c77768a86216be456c2764e32a2bcd841

needed to be reverted with:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=ad3ccd760da7c05b90775372f9b39dc2964086fe

Because nested plugs caused generic_make_request()'s onstack bio_list to
go out of scope (blk_finish_plug() wouldn't actually flush the list
within generic_make_request because XFS already added an outermost
plug).

But even after fixing that I then hit issues with these changes now
resulting in imperfect 'in_generic_make_request' accounting that happens
lazily once the outermost plug completes blk_finish_plug.  manifested as
dm-bufio.c:dm_bufio_prefetch's BUG_ON(dm_bufio_in_request()); hitting.

Basically using the blk-core's onstack plugging isn't workable for
fixing this deadlock and we're back to having to seriously consider
this (with its additional hook in the scheduler): 

Mike