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

[Vladimir Sementsov-Ogievskiy]

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