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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
