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

[Andrey Shinkevich]

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.

Andrey

> 
> I don't think that's a case we want to just forbid because nobody needs
> this anyway. It's a sign of a more fundamental problem in our design,
> and I'm sure it will bite us in other places, too.
> 
> Kevin
>