Re: [PATCH 10/12] block.c: add subtree_drains where needed

[Paolo Bonzini]

On 2/2/22 16:37, Emanuele Giuseppe Esposito wrote:
> So we have disk B with backing file C, and new disk A that wants to have
> backing file C.
>
> I think I understand what you mean, so in theory the operation would be
> - create new child
> - add child to A->children list
> - add child to C->parents list
>
> So in theory we need to
> * drain A (without subtree), because it can't happen that child nodes of
>    A have in-flight requests that look at A status (children list), right?
>    In other words, if A has another node X, can a request in X inspect
>    A->children
> * drain C, as parents can inspect C status (like B). Same assumption
>    here, C->children[x]->bs cannot have requests inspecting C->parents
>    list?

In that case (i.e. if parents have to be drained, but children need not) 
bdrv_drained_begin_unlocked would be enough, right?

That would mean that ->children is I/O state but ->parents is global 
state.  I think it's quite a bit more complicated to analyze and to 
understand.

Paolo