ping ping!!! On 18.12.2018 11:53, Denis Plotnikov wrote: > 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
