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[Qemu-block] PING: [PATCH] blk: postpone request execution on a context

From: Denis Plotnikov
Subject: [Qemu-block] PING: [PATCH] blk: postpone request execution on a context protected with "drained section"
Date: Thu, 17 Jan 2019 12:57:45 +0000


could you please take a look at my last comments?



On 15.01.2019 10:22, Denis Plotnikov wrote:
> ping ping ping ping!!!!
> On 09.01.2019 11:18, Denis Plotnikov wrote:
>> ping ping!!!
>> On 18.12.2018 11:53, Denis Plotnikov wrote:
>>> ping ping
>>> On 14.12.2018 14:54, Denis Plotnikov wrote:
>>>> On 13.12.2018 15:20, Kevin Wolf wrote:
>>>>> Am 13.12.2018 um 12:07 hat Denis Plotnikov geschrieben:
>>>>>> On 12.12.2018 15:24, Kevin Wolf wrote:
>>>>>>> Am 11.12.2018 um 17:55 hat Denis Plotnikov geschrieben:
>>>>>>>>> Why involve the AioContext at all? This could all be kept at the
>>>>>>>>> BlockBackend level without extending the layering violation that
>>>>>>>>> aio_disable_external() is.
>>>>>>>>> BlockBackends get notified when their root node is drained, so 
>>>>>>>>> hooking
>>>>>>>>> things up there should be as easy, if not even easier than in
>>>>>>>>> AioContext.
>>>>>>>> Just want to make sure that I understood correctly what you 
>>>>>>>> meant by
>>>>>>>> "BlockBackends get notified". Did you mean that bdrv_drain_end 
>>>>>>>> calls
>>>>>>>> child's role callback blk_root_drained_end by calling
>>>>>>>> bdrv_parent_drained_end?
>>>>>>> Yes, blk_root_drained_begin/end calls are all you need. 
>>>>>>> Specifically,
>>>>>>> their adjustments to blk->quiesce_counter that are already there, 
>>>>>>> and in
>>>>>>> the 'if (--blk->quiesce_counter == 0)' block of 
>>>>>>> blk_root_drained_end()
>>>>>>> we can resume the queued requests.
>>>>>> 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...
>>>> In mirror_exit_common() we actively manipulate with block driver 
>>>> states.
>>>> When we replaced a node in the snippet you showed we can't allow the 
>>>> postponed coroutines to run because the block tree isn't ready to 
>>>> receive the requests yet.
>>>> To be ready, we need to insert a proper block driver state to the 
>>>> block backend which is done here
>>>>      blk_remove_bs(bjob->blk);
>>>>      blk_set_perm(bjob->blk, 0, BLK_PERM_ALL, &error_abort);
>>>>      blk_insert_bs(bjob->blk, mirror_top_bs, &error_abort); << << << <<
>>>>      bs_opaque->job = NULL;
>>>>      bdrv_drained_end(src);
>>>> If the tree isn't ready and we resume the coroutines, we'll end up 
>>>> with the request landed in a wrong block driver state.
>>>> So, we explicitly should stop all activities on all the driver states
>>>> and its parents and allow the activities when everything is ready to 
>>>> go.
>>>> Why explicitly, because the block driver states may belong to 
>>>> different block backends at the moment of the manipulation beginning.
>>>> So, it seems we need to disable all their contexts until the 
>>>> manipulation ends.
>>>> Please, correct me if I'm wrong.
>>>>>> In seems that if the external requests disabled on the context we 
>>>>>> can't
>>>>>> rely on anything or should check where the underlying bs and its
>>>>>> underlying nodes are ready to receive requests which sounds quite
>>>>>> complicated.
>>>>>> Please correct me if still don't understand something in that 
>>>>>> routine.
>>>>> I think the reason why reyling on aio_disable_external() works is 
>>>>> simply
>>>>> because src is also drained, which keeps external events in the
>>>>> AioContext disabled despite the bug in draining the target node.
>>>>> The bug would become apparent even with aio_disable_external() if we
>>>>> didn't drain src, or even if we just supported src and target being in
>>>>> different AioContexts.
>>>> Why don't we disable all those contexts involved until the end of 
>>>> the block device tree reconstruction?
>>>> Thanks!
>>>> Denis
>>>>> Kevin


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