[dm-devel] [PATCH/rfc] dm: revise 'rescue' strategy for md->bs allocations

[Mikulas Patocka]

On Fri, 1 Sep 2017, NeilBrown wrote:

> On Thu, Aug 31 2017, Mikulas Patocka wrote:
> 
> >> 
> >> Note that only current->bio_list[0] is offloaded.  current->bio_list[1]
> >> contains bios that were scheduled *before* the current one started, so
> >
> > These bios need to be offloaded too, otherwise you re-introduce this 
> > deadlock: https://www.redhat.com/archives/dm-devel/2014-May/msg00089.html
> 
> Thanks for the link.
> In the example the bio that is stuck was created in step 4.  The call
> to generic_make_request() will have placed it on current->bio_list[0].
> The top-level generic_make_request call by Process A is still running,
> so nothing will have moved the bio to ->bio_list[1].  That only happens
> after the ->make_request_fn completes, which must be after step 7.
> So the problem bio is on ->bio_list[0] and the code in my patch will
> pass it to a workqueue for handling.
> 
> So I don't think the deadlock would be reintroduced.  Can you see
> something that I am missing?
> 
> Thanks,
> NeilBrown

Offloading only current->bio_list[0] will work in a simple case described 
above, but not in the general case.

For example, suppose that we have a dm device where the first part is 
linear and the second part is snapshot.

* We receive bio A that crosses the linear/snapshot boundary
* DM core creates bio B, submits it to the linear target and adds it to 
current->bio_list[0]
* DM core creates bio C, submits it to the snapshot target, the snapshot 
target calls track_chunk for this bio and appends the bio to 
current->bio_list[0]
* Now, we return back to generic_make_request
* We pop bio B from current->bio_list[0]
* We execute bio_list_on_stack[1] = bio_list_on_stack[0], that moves bio C 
to bio_list_on_stack[1] - and now we lose any possibility to offload bio C 
to the rescue thread
* The kcopyd thread for the snapshot takes the snapshot lock and waits for 
bio C to finish
* We process bio B - and if processing bio B reaches something that takes 
the snapshot lock (for example an origin target for the snapshot), a 
deadlock will happen. The deadlock could be avoided by offloading bio C to 
the rescue thread, but bio C is already on bio_list_on_stack[1] and so it 
won't be offloaded

Mikulas