Re: [RFC PATCH 0/3] block: Synchronous bdrv_*() from coroutine in different AioContext

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

On 5/12/20 4:43 PM, Kevin Wolf wrote:
> Stefan (Reiter), after looking a bit closer at this, I think there is no
> bug in QEMU, but the bug is in your coroutine code that calls block
> layer functions without moving into the right AioContext first. I've
> written this series anyway as it potentially makes the life of callers
> easier and would probably make your buggy code correct.

> However, it doesn't feel right to commit something like patch 2 without
> having a user for it. Is there a reason why you can't upstream your
> async snapshot code?

I mean I understand what you mean, but it would make the interface IMO so
much easier to use, if one wants to explicit schedule it beforehand they
can still do. But that would open the way for two styles doing things, not
sure if this would seen as bad. The assert about from patch 3/3 would be
already really helping a lot, though.

Regarding upstreaming, there was some historical attempt to upstream it
from Dietmar, but in the time frame of ~ 8 to 10 years ago or so.
I'm not quite sure why it didn't went through then, I see if I can get some
time searching the mailing list archive.

We'd be naturally open and glad to upstream it, what it effectively allow
us to do is to not block the VM to much during snapshoting it live.

I pushed a tree[0] with mostly just that specific code squashed together (hope
I did not break anything), most of the actual code is in commit [1].
It'd be cleaned up a bit and checked for coding style issues, but works good
here.

Anyway, thanks for your help and pointers!

[0]: https://github.com/ThomasLamprecht/qemu/tree/savevm-async
[1]: https://github.com/ThomasLamprecht/qemu/commit/ffb9531f370ef0073e4b6f6021f4c47ccd702121