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Re: [PATCH 0/7] block: Attempt on fixing 030-reported errors

From: Hanna Reitz
Subject: Re: [PATCH 0/7] block: Attempt on fixing 030-reported errors
Date: Thu, 4 Nov 2021 14:34:41 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

On 04.11.21 12:58, Kevin Wolf wrote:
Am 04.11.2021 um 11:38 hat Hanna Reitz geschrieben:
(2A) bdrv_replace_child_noperm() should immediately set bs->file or
      bs->backing to NULL when it sets bs->{file,backing}->bs to NULL.
      It should also immediately remove any BdrvChild with .bs == NULL
      from the parent’s BDS.children list.
      Implemented in patches 2 through 6.

(2B) Alternatively, we could always keep the whole subgraph drained
      while we manipulate it.  Then, the bdrv_parent_drained_end_single()
      in bdrv_replace_child_noperm() wouldn’t do anything.
      To fix 030, we would need to add a drained section to
      stream_prepare(): Namely we’d need to drain the subgraph below the
      COR filter node.
      This would be a much simpler solution, but I don’t feel like it’s
      the right one.
As you can see, I’m not sure which of 2A or 2B is the right solution.  I
decided to investigate both: 2A was much more complicated, but seemed
like the right thing to do; 2B is much simpler, but doesn’t feel as
right.  Therefore, I decided to go with 2A in this first version of this
I haven't looked at the patches yet, but if I understand correctly the
choice you're presenting here is between protecting code from accessing
invalid state and not creating the invalid state in the first place.

Yes, that’s right.

I agree that the latter is preferable as long as it doesn't make things
so complicated that we would be willing to accept the higher risk of
breakage in the former.

No, I don’t think it’s too complicated.  Just not as sample as a drained_begin + drained_end.

If it's doable in five patches, it's probably
not complicated enough to make such compromises.

Without the clean-up patches that are patches 3 and 4, it would be doable in even fewer patches. :)


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