Hello, Kent. On Mon, Sep 10, 2012 at 02:56:33PM -0700, Kent Overstreet wrote: > commit df7e63cbffa3065fcc4ba2b9a93418d7c7312243 > Author: Kent Overstreet <koverstreet@xxxxxxxxxx> > Date: Mon Sep 10 14:33:46 2012 -0700 > > block: Avoid deadlocks with bio allocation by stacking drivers > > Previously, if we ever try to allocate more than once from the same bio > set while running under generic_make_request() (i.e. a stacking block > driver), we risk deadlock. > > This is because of the code in generic_make_request() that converts > recursion to iteration; any bios we submit won't actually be submitted > (so they can complete and eventually be freed) until after we return - > this means if we allocate a second bio, we're blocking the first one > from ever being freed. > > Thus if enough threads call into a stacking block driver at the same > time with bios that need multiple splits, and the bio_set's reserve gets > used up, we deadlock. > > This can be worked around in the driver code - we could check if we're > running under generic_make_request(), then mask out __GFP_WAIT when we > go to allocate a bio, and if the allocation fails punt to workqueue and > retry the allocation. > > But this is tricky and not a generic solution. This patch solves it for > all users by inverting the previously described technique. We allocate a > rescuer workqueue for each bio_set, and then in the allocation code if > there are bios on current->bio_list we would be blocking, we punt them > to the rescuer workqueue to be submitted. > > This guarantees forward progress for bio allocations under > generic_make_request() provided each bio is submitted before allocating > the next, and provided the bios are freed after they complete. > > Note that this doesn't do anything for allocation from other mempools. > Instead of allocating per bio data structures from a mempool, code > should use bio_set's front_pad. > > Tested it by forcing the rescue codepath to be taken (by disabling the > first GFP_NOWAIT) attempt, and then ran it with bcache (which does a lot > of arbitrary bio splitting) and verified that the rescuer was being > invoked. > > Signed-off-by: Kent Overstreet <koverstreet@xxxxxxxxxx> > CC: Jens Axboe <axboe@xxxxxxxxx> I'm still a bit scared but think this is correct. Acked-by: Tejun Heo <tj@xxxxxxxxxx> One last thing is that we may want to add @name on bioset creation so that we can name the workqueue properly but that's for another patch. Thanks. -- tejun -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
