Re: new patchset to eliminate DM's use of BIOSET_NEED_RESCUER

[Mike Snitzer]

On Tue, Nov 21 2017 at  4:23pm -0500,
Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:

> 
> 
> On Tue, 21 Nov 2017, Mike Snitzer wrote:
> 
> > On Tue, Nov 21 2017 at  7:43am -0500,
> > Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> >  
> > > Decided it a better use of my time to review and then hopefully use the
> > > block-core's bio splitting infrastructure in DM.  Been meaning to do
> > > that for quite a while anyway.  This mail from you just made it all the
> > > more clear that needs doing:
> > > https://www.redhat.com/archives/dm-devel/2017-September/msg00098.html
> > > 
> > > So I will start here on this patch you proposed:
> > > https://www.redhat.com/archives/dm-devel/2017-September/msg00091.html
> > > (of note, this patch slipped through the cracks because I was recovering
> > > from injury when it originally came through).
> > > 
> > > Once DM is using q->bio_split I'll come back to this patch (aka
> > > "[1]") as a starting point for the follow-on work to remove DM's use of
> > > BIOSET_NEED_RESCUER:
> > > https://www.redhat.com/archives/dm-devel/2017-August/msg00315.html
> > 
> > Hey Neil,
> > 
> > Good news!  All your code works ;)
> > 
> > (well after 1 fixup due to a cut-n-paste bug.. the code you added to
> > dm_wq_work() to process the md->rescued bio_list was operating on
> > the md->deferred bio_list due to cut-n-paste from code you copied from
> > just below it)
> > 
> > I split your code out some to make it more reviewable.  I also tweaked
> > headers accordingly.
> > 
> > Please see this branch (which _will_ get rebased between now and the
> > 4.16 merge window):
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-4.16
> > 
> > I successfully tested these changes using Mikulas' test program that
> > reproduces the snapshot deadlock:
> > https://www.redhat.com/archives/dm-devel/2017-January/msg00064.html
> > 
> > I'll throw various other DM testsuites at it to verify they all look
> > good (e.g. thinp, cache, multipath).
> > 
> > I'm open to all suggestions about changes you'd like to see (either to
> > these patches or anything you'd like to layer ontop of them).
> > 
> > Thanks for all your work, much appreciated!
> > Mike
> 
> This is not correct:
> 
>    2206 static void dm_wq_work(struct work_struct *work)
>    2207 {
>    2208         struct mapped_device *md = container_of(work, struct mapped_device, work);
>    2209         struct bio *bio;
>    2210         int srcu_idx;
>    2211         struct dm_table *map;
>    2212
>    2213         if (!bio_list_empty(&md->rescued)) {
>    2214                 struct bio_list list;
>    2215                 spin_lock_irq(&md->deferred_lock);
>    2216                 list = md->rescued;
>    2217                 bio_list_init(&md->rescued);
>    2218                 spin_unlock_irq(&md->deferred_lock);
>    2219                 while ((bio = bio_list_pop(&list)))
>    2220                         generic_make_request(bio);
>    2221         }
>    2222
>    2223         map = dm_get_live_table(md, &srcu_idx);
>    2224
>    2225         while (!test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags)) {
>    2226                 spin_lock_irq(&md->deferred_lock);
>    2227                 bio = bio_list_pop(&md->deferred);
>    2228                 spin_unlock_irq(&md->deferred_lock);
>    2229
>    2230                 if (!bio)
>    2231                         break;
>    2232
>    2233                 if (dm_request_based(md))
>    2234                         generic_make_request(bio);
>    2235                 else
>    2236                         __split_and_process_bio(md, map, bio);
>    2237         }
>    2238
>    2239         dm_put_live_table(md, srcu_idx);
>    2240 }
> 
> You can see that if we are in dm_wq_work in __split_and_process_bio, we 
> will not process md->rescued list.

Can you elaborate further?  We cannot be "in dm_wq_work in
__split_and_process_bio" simultaneously.  Do you mean as a side-effect
of scheduling away from __split_and_process_bio?

The more detail you can share the better.

> The processing of md->rescued is also wrong - bios for different devices 
> must be offloaded to different helper threads, so that processing a bio 
> for a lower device doesn't depend on processing a bio for a higher device. 
> If you offload all the bios on current->bio_list to the same thread, the 
> bios still depend on each other and the deadlock will still happen.

Commit 325738403 ("dm: revise 'rescue' strategy for bio-based bioset
allocations") speaks to this with:

"Note that only current->bio_list[0] is offloaded.  current->bio_list[1]
contains bios that were scheduled *before* the current one started, so
they must have been submitted from higher up the stack, and we cannot be
waiting for them here (thanks to the "dm: ensure bio submission follows
a depth-first tree walk" commit).  Also, we now rescue *all* bios on the
list as there is nothing to be gained by being more selective."

And again: this patchset passes your dm-snapshot deadlock test.  Is
that test somehow lacking?

Or do you see a hypothetical case where a deadlock is still possible?
That is of less concern.  I'd prefer that we tackle problems for
targets, and associated scenarios, that we currently support.

Either way, happy to review this with you further.  Any fixes are
welcomed too.  But I'd like us to head in a direction that this patchset
is taking us.  Specifically: away from DM relying on BIOSET_NEED_RESCUER.

Thanks,
Mike