Re: [Qemu-devel] [PATCH 1/2] iotests: 030 TestParallelOps non-shared base node

[Vladimir Sementsov-Ogievskiy]

On 21.03.2019 20:05, Andrey Shinkevich wrote:
> 
> 
> On 21/03/2019 13:53, Kevin Wolf wrote:
>> Am 20.03.2019 um 18:02 hat Alberto Garcia geschrieben:
>>> On Wed 20 Mar 2019 10:16:10 AM CET, Kevin Wolf wrote:
>>>>> Oh, I see. Let's use a shorter chain for simplicity:
>>>>>
>>>>>      A <- B <- C <- D <- E
>>>>
>>>> Written from right to left, i.e. E being the base and A the top layer?
>>>> We usually write things the other write round, I hope this doesn't get
>>>> too confusing later.
>>>
>>> Oh my... yes, of course you're right, I should have written it the other
>>> way around:
>>>
>>>      E <- D <- C <- B <- A
>>>
>>>>> 1) If we stream first from E to C we add a filter F:
>>>>>
>>>>>      A <- B <- F <- C <- D <- E
>>>
>>> ( which should have been written   E <- D <- C <- F <- B <- A )
>>>
>>>>>      Now we can't stream from C to A because F is on the way, and the F-C
>>>>>      link is frozen.
>>>>
>>>> Why is a frozen link a problem? The streaming operation isn't going to
>>>> change this link, it just copies data from the subchain (including F
>>>> and C) to A. This is not something that a frozen link should prevent.
>>>
>>> Not the operation itself, but the first thing that block-stream does is
>>> freeze the chain from top (A) to base (C), so this would fail if there's
>>> already a frozen link on the way (C <- F on this case?).
>>
>> Oh, I see. I think this is why I suggested a counter originally instead
>> of a bool.
>>
>>>> So it seems frozen links allow the wrong case, but block the correct
>>>> one? :-(
>>>
>>> Yes, we probably need to rethink this scenario a bit.
>>
>> But yes, even with a counter, the other problem would still remain (that
>> the first job can't remove the filter on completion because the second
>> one has frozen its link to the filter).
> 
> With this example E <- D <- C <- F <- B <- A,
> 
> In the current implementation of the copy-on-read filter,
> its bs->backing is not initialized (while it is not true for the filter
> in block-commit job). So, bdrv_freeze_backing_chain() doesn't go beyond
> the cor-filter node. With the two parallel block-stream jobs, we get the
> following sub-chains frozen:
> F <- B <- A
> E <- D <- C
> as C <- F backing BdrvChild link doesn't exist.
> If the cor-filter is inserted with the bdrv_append(), we get two
> BdrvChild links (file and backing) pointed to the same BlockDriverState
> 'C' and additionally some child-permissions issues that I have not
> resolved yet...
> Due to the fact mentioned above, freezing the backing chain works with
> the filter inserted. But, with the one BdrvChild *file link only in the
> BlockDriverState of the cor-filter, we encounter a broken chain each
> time we iterate through it with the backing_bs(F) (=NULL) in many other
> pieces of the code. In turn, it breaks the existing model.
> That's the point! (((
> What can we do with that?
> 
> In my patch Stream-block-job-involves-copy-on-read-filter.patch :
> 
> static BlockDriverState *child_file_bs(BlockDriverState *bs)
> {
>       return bs->file ? bs->file->bs : NULL;
> }
> 
> static BlockDriverState *skip_filter(BlockDriverState *bs)
> {
>       BlockDriverState *ret_bs = bs;
> 
>       while (ret_bs && ret_bs->drv && ret_bs->drv->is_filter) {
>           ret_bs = child_file_bs(ret_bs);
>       }
> 
>       return ret_bs;
> }
> 
> But the solution above looks clumsy to me.
> I would appreciate to hear any other ideas from you.
> 

As I understand, we can't drop .file child from copy-on-read and use 
backing instead as it'll break backward compatibility. So I propose to
support backing child in COR, so that it may operate through .file or
through .backing, depending on what it has. (But forbid creating both
children)