On 21.06.2019 12:59, Vladimir Sementsov-Ogievskiy wrote: > 21.06.2019 12:16, Kevin Wolf wrote: >> Am 09.04.2019 um 12:01 hat Kevin Wolf geschrieben: >>> Am 02.04.2019 um 10:35 hat Denis Plotnikov geschrieben: >>>> On 13.03.2019 19:04, Kevin Wolf wrote: >>>>> Am 14.12.2018 um 12:54 hat Denis Plotnikov geschrieben: >>>>>> On 13.12.2018 15:20, Kevin Wolf wrote: >>>>>>> Am 13.12.2018 um 12:07 hat Denis Plotnikov geschrieben: >>>>>>>> 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... >>>>> >>>>> Did you ever implement the changes suggested so far, so that we could >>>>> continue from there? Or should I try and come up with something myself? >>>> >>>> Sorry for the late reply... >>>> Yes, I did ... >>> >>> If there are more question or problems, can you post the patches in >>> their current shape (as an RFC) or a git URL so I can play with it a >>> bit? If you could include a failing test case, too, that would be ideal. >> >> Denis? Please? >> >> We really should get this fixed and I would be willing to lend a hand, >> but if you keep your patches secret, I can't really do so and would have >> to duplicate your work. >> >> Also, please see my old answer from April below for the last problem you >> had with implementing the correct approach. >> >> Kevin
Hi Kevin, I'm sorry for not replying for so long. Please, give me some time (a day or two) so I could refresh everything and send the current state of the patches as well as the test case checking the issue Denis > > He is not at work today, I think he'll be able to answer on Monday. > > -- Best, Denis
