ping ping On 14.12.2018 14:54, Denis Plotnikov wrote: > > > On 13.12.2018 15:20, Kevin Wolf wrote: >> Am 13.12.2018 um 12:07 hat Denis Plotnikov geschrieben: >>> On 12.12.2018 15:24, Kevin Wolf wrote: >>>> Am 11.12.2018 um 17:55 hat Denis Plotnikov geschrieben: >>>>>> Why involve the AioContext at all? This could all be kept at the >>>>>> BlockBackend level without extending the layering violation that >>>>>> aio_disable_external() is. >>>>>> >>>>>> BlockBackends get notified when their root node is drained, so >>>>>> hooking >>>>>> things up there should be as easy, if not even easier than in >>>>>> AioContext. >>>>> >>>>> Just want to make sure that I understood correctly what you meant by >>>>> "BlockBackends get notified". Did you mean that bdrv_drain_end calls >>>>> child's role callback blk_root_drained_end by calling >>>>> bdrv_parent_drained_end? >>>> >>>> Yes, blk_root_drained_begin/end calls are all you need. Specifically, >>>> their adjustments to blk->quiesce_counter that are already there, >>>> and in >>>> the 'if (--blk->quiesce_counter == 0)' block of blk_root_drained_end() >>>> we can resume the queued requests. >>> Sounds it should be so, but it doesn't work that way and that's why: >>> when doing mirror we may resume postponed coroutines too early when the >>> underlying bs is protected from writing at and thus we encounter the >>> assert on a write request execution at bdrv_co_write_req_prepare when >>> resuming the postponed coroutines. >>> >>> The thing is that the bs is protected for writing before execution of >>> bdrv_replace_node at mirror_exit_common and bdrv_replace_node calls >>> bdrv_replace_child_noperm which, in turn, calls child->role->drained_end >>> where one of the callbacks is blk_root_drained_end which check >>> if(--blk->quiesce_counter == 0) and runs the postponed requests >>> (coroutines) if the coundition is true. >> >> Hm, so something is messed up with the drain sections in the mirror >> driver. We have: >> >> bdrv_drained_begin(target_bs); >> bdrv_replace_node(to_replace, target_bs, &local_err); >> bdrv_drained_end(target_bs); >> >> Obviously, the intention was to keep the BlockBackend drained during >> bdrv_replace_node(). So how could blk->quiesce_counter ever get to 0 >> inside bdrv_replace_node() when target_bs is drained? >> >> Looking at bdrv_replace_child_noperm(), it seems that the function has >> a bug: Even if old_bs and new_bs are both drained, the quiesce_counter >> for the parent reaches 0 for a moment because we call .drained_end for >> the old child first and .drained_begin for the new one later. >> >> So it seems the fix would be to reverse the order and first call >> .drained_begin for the new child and then .drained_end for the old >> child. Sounds like a good new testcase for tests/test-bdrv-drain.c, too. > Yes, it's true, but it's not enough... > In mirror_exit_common() we actively manipulate with block driver states. > When we replaced a node in the snippet you showed we can't allow the > postponed coroutines to run because the block tree isn't ready to > receive the requests yet. > To be ready, we need to insert a proper block driver state to the block > backend which is done here > > blk_remove_bs(bjob->blk); > blk_set_perm(bjob->blk, 0, BLK_PERM_ALL, &error_abort); > blk_insert_bs(bjob->blk, mirror_top_bs, &error_abort); << << << << > > bs_opaque->job = NULL; > > bdrv_drained_end(src); > > If the tree isn't ready and we resume the coroutines, we'll end up with > the request landed in a wrong block driver state. > > So, we explicitly should stop all activities on all the driver states > and its parents and allow the activities when everything is ready to go. > > Why explicitly, because the block driver states may belong to different > block backends at the moment of the manipulation beginning. > > So, it seems we need to disable all their contexts until the > manipulation ends. > > Please, correct me if I'm wrong. > >> >>> In seems that if the external requests disabled on the context we can't >>> rely on anything or should check where the underlying bs and its >>> underlying nodes are ready to receive requests which sounds quite >>> complicated. >>> Please correct me if still don't understand something in that routine. >> >> I think the reason why reyling on aio_disable_external() works is simply >> because src is also drained, which keeps external events in the >> AioContext disabled despite the bug in draining the target node. >> >> The bug would become apparent even with aio_disable_external() if we >> didn't drain src, or even if we just supported src and target being in >> different AioContexts. > > Why don't we disable all those contexts involved until the end of the > block device tree reconstruction? > > Thanks! > > Denis >> >> Kevin >> >
-- Best, Denis