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Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption
Date: Fri, 29 Jun 2012 10:24:24 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0

Am 29.06.2012 02:50, schrieb Peter Crosthwaite:
>> The problem I see is:
>> FDTMachineInfo *fdt_generic_create_machine()
>> {
>>    ...
>>    while (qemu_co_queue_enter_next(fdti->cq));
>> }
>> The problem you have is not that the block layer is yielding.  The
>> problem is that you need to run aio processing
> And this is what Im trying to say is wrong. My usage of coroutine has
> nothing to do with block, AIO or any other existing client of
> coroutine. I shouldnt have to make API calls into things I dont care
> about just to make my coroutines work. The QOM people should be able
> to come along and rewrite the AIO subsystem tommorow and it shouldnt
> affect my coroutines.

This isn't really block or AIO specific. What you need to run is
basically a nested main loop. As long as you process the
(aio_)fd_handlers and bottom halves, the block layer will work.
qemu_aio_wait() implements such a nested main loop. I think the only
major thing that is not executed in qemu_aio_wait() is timers (and maybe
that's something we should change).

> Maybe I need to generalise away from block. This problem goes a level
> higher to AIO. AIO is assuming it owns all coroutines - I.E. if you
> are in coroutines context (qemu_in_coroutine()) then that coroutine
> can be pre-emented and queued up in AIO's work queue. Im saying this
> is flawed - because it means coroutines are not generic, they are for
> AIO use only.
> So change subject to "Remove assumption that AIO owns coroutines".

No, the assumption is a completely different one and it has nothing to
do with who "owns" a coroutine. It is about whether you may assume that
a call into a different subsystem doesn't yield. The answer today is no,
the other subsystem may yield. You want to change it to yes. But there's
nothing AIO or block specific about it.


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