Re: [Qemu-devel] [Qemu-block] [PATCH] blk: postpone request execution on a context protected with "drained section"

[Denis Plotnikov]

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