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 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. Please take a look. I'm ready to discuss the ways to improve it and will reply as fast as I can. Thanks! Denis > > Denis >> >> He is not at work today, I think he'll be able to answer on Monday. >> >> > -- Best, Denis
