Re: [PATCH v2 00/15] block: Simplify drain

[Kevin Wolf <kwolf@redhat.com>]

Am 18.11.2022 um 18:40 hat Kevin Wolf geschrieben:
> I'm aware that exactly nobody has been looking forward to a series with
> this title, but it has to be. The way drain works means that we need to
> poll in bdrv_replace_child_noperm() and that makes things rather messy
> with Emanuele's multiqueue work because you must not poll while you hold
> the graph lock.
> 
> The other reason why it has to be is that drain is way too complex and
> there are too many different cases. Some simplification like this will
> hopefully make it considerably more maintainable. The diffstat probably
> tells something, too.
> 
> There are roughly speaking three parts in this series:
> 
> 1. Make BlockDriver.bdrv_drained_begin/end() non-coroutine_fn again,
>    which allows us to not poll on bdrv_drained_end() any more.
> 
> 2. Remove subtree drains. They are a considerable complication in the
>    whole drain machinery (in particular, they require polling in the
>    BdrvChildClass.attach/detach() callbacks that are called during
>    bdrv_replace_child_noperm()) and none of their users actually has a
>    good reason to use them.
> 
> 3. Finally get rid of polling in bdrv_replace_child_noperm() by
>    requiring that the child is already drained by the caller and calling
>    callbacks only once and not again for every nested drain section.
> 
> If necessary, a prefix of this series can be merged that covers only the
> first or the first two parts and it would still make sense.

Thanks for the review, applied to block-next.

Kevin