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Re: [Qemu-block] [Qemu-devel] [PATCH] blk: postpone request execution on

From: Denis Plotnikov
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] blk: postpone request execution on a context protected with "drained section"
Date: Tue, 2 Jul 2019 14:41:23 +0000

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.

>> 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


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