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Re: [Qemu-devel] [Qemu-block] Block layer complexity: what to do to keep
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [Qemu-block] Block layer complexity: what to do to keep it under control? |
Date: |
Wed, 29 Nov 2017 13:24:55 +0000 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Wed, Nov 29, 2017 at 01:24:46PM +0100, Paolo Bonzini wrote:
> On 29/11/2017 13:00, Stefan Hajnoczi wrote:
> > We are at a point where code review isn't finding certain bugs because
> > no single person knows all the assumptions. Previously the problem was
> > contained because maintainers spotted problems before patches were
> > merged.
> >
> > This is not primarily a documentation problem though. We cannot
> > document our way out of this because no single person (patch author or
> > code reviewer) can know or check everything anymore due to the scale.
> >
> > I think it's a (lack of) design problem because we have many incomplete
> > abstractions like block jobs, IOThreads, block graph, image locking,
> > etc. They do not cover all possibly states and interactions today.
> > Extending them leads to complex bugs.
>
> I think the main interactions are:
>
> 1) block graph modifications and drain. This has always been a carnage.
> Implementing BlockBackend isolation instead of drain would probably be
> a starting point to fix it, because IIRC there are extremely few cases
> where we really need "drain" semantics.
Yes, this is a big one.
> 2) block jobs and coroutines. Block jobs were too clever about
> coroutines. Using a simplified API is going to fix this problem.
> Ideally, if you're not in a coroutine "co", the only coroutine APIs you
> should use on "co" are:
>
> - aio_co_enter/qemu_coroutine_enter (start a coroutine, respectively on
> another AioContext or this context);
>
> - aio_co_schedule/aio_co_wake (restart a coroutine that has yielded,
> respectively on a given AioContext or its own original.
>
> 3) block jobs and drain. This is related to (1) because drain can
> terminate jobs and in turn that can cause block graph modifications.
> I'm not even sure it's a separate issue.
>
>
> Regarding documentation, the include file documentation is good for
> coroutines and block jobs. But it's bad for block graph modification
> APIs, and even for coroutines + block jobs the docs/devel documentation
> could be improved *and* it's ugly that we're not generating anything
> readable from include file documentation, to go with docs/devel.
We still need to be careful about block job completion code that runs in
the main loop, too. That has been a source of several bugs.
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