On 28.06.2019 15:32, Kevin Wolf wrote: > Am 26.06.2019 um 10:46 hat Denis Plotnikov geschrieben: >> On 24.06.2019 12:46, Denis Plotnikov wrote: >>> 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 >> >> Hi Kevin, >> The current state of the patches is available at >> https://github.com/denis-plotnikov/qemu/tree/postponed-request > > Are you sure you pushed the correct version? > > I don't see any of the things we discussed above in this branch, i.e. > using blk_root_drained_begin/end, fixing bdrv_replace_child_noperm(), > fixing the drain calls in mirror etc. I didn't include them intentionally because I didn't manage to make them work. I just stick with something that work more or less ok. If you want, I can do the related modifications in a separate brunch so you can try them by yourself.
Denis > >> I didn't manage to create an automatic reproducer but one of the patches >> contains a step-by-step description of how to reproduce the problem. > > Ok, I'll try whether I can reproduce this. > > Kevin > -- Best, Denis
