On Thu, 13 Aug 2015, NeilBrown wrote: > On Wed, 12 Aug 2015 12:25:42 -0400 (EDT) Mikulas Patocka > <mpatocka@xxxxxxxxxx> wrote: > > > > > > > On Wed, 12 Aug 2015, NeilBrown wrote: > > > > > On Tue, 11 Aug 2015 05:14:33 -0400 (EDT) Mikulas Patocka > > > <mpatocka@xxxxxxxxxx> wrote: > > > > > > > Hi > > > > > > > > On Mon, 10 Aug 2015, NeilBrown wrote: > > > > > > > > > > > > > > Hi Mikulas, > > > > > I have a customer hitting the deadlock you described over a year ago > > > > > in: > > > > > > > > > > Subject: [PATCH] block: flush queued bios when the process > > > > > blocks > > > > > > > > Ask block layer maintainers to accept that patch. > > > > > > Unfortunately I don't really like the patch ... or the bioset rescue > > > workqueues that it is based on. Sorry. > > > > > > So I might keep looking for a more localised approach.... > > > > The problem here is that other dm targets may deadlock in a similar way > > too - for example, dm-thin could deadlock on pmd->pool_lock. > > > > The cause of the bug is bio queuing on current->bio_list. There is an > > assumption that if a dm target submits a bio to a lower-level target, the > > bio finishes in finite time. Queuing on current->bio_list breaks the > > assumption, bios can be held indefinitelly on current->bio_list. > > > > The patch that flushes current->bio_list is the correct way to fix it - it > > makes sure that a bio can't be held indefinitely. > > > > Another way to fix it would be to abandon current->bio_list --- but then, > > there would be problem with stack overflow on deeply nested targets. > > > > I think it is a bit simplistic to say that current->bio_list is the > "cause" of the problem. It is certainly a part of the problem. The > assumption you mention is another part - and the two conflict. > > As you say, we could abandon current->bio_list, but then we risk stack > overflows again. > Or we could hand the bio_list to a work-queue whenever the make_request > function needs to schedule.... but then if handling one of those bios > needs to schedule... not sure, it might work. > > Or we could change the assumption and never block in a make_request > function after calling generic_make_request(). Maybe that is difficult. > > Or we could change __split_and_process_bio to use bio_split() to split > the bio, then handle the first and call generic_make_request on the > second. That would effectively put the second half on the end of > bio_list so it wouldn't be tried until all requests derived from the > first half have been handled. I don't think it will fix the bug - even if you put the second half of the bio at the end of bio_list, it will still wait until other entries on the bio list are processed. For example - device 1 gets a bio, splits it to bio1 and bio2, forwards them to device 2 and put them on current->bio_list Device 2 receives bio1 (popped from curretn->bio_list), splits it to bio3 and bio4, forwards them to device 3 and puts them at the end of current->bio_list Device 2 receives bio2 (popped from current->bio_list), waits until bio1 finishes, but bio1 won't ever finish because it depends on bio3 and bio4 that are on current->bio_list. > None of these is completely straight forward, but I suspect all are > possible. > > I'm leaning towards the last one: when you want to split a bio, use > bio_split and handle the two halves separately. > > Do you have thoughts on that? > > Thanks, > NeilBrown Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
