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
He is not at work today, I think he'll be able to answer on Monday. -- Best regards, Vladimir
