Re: [Qemu-block] [PATCH for-2.11 3/4] coroutine: Cancel aio_co_schedule() on direct entry

[Paolo Bonzini]

On 28/11/2017 18:09, Jeff Cody wrote:
>> Yes, terminating a scheduled coroutine is a bug; same for scheduling a
>> terminated coroutine, both orders are wrong. However, "unscheduling" is
>> not the solution; you would just be papering over the issue.
>
> Maybe we should at least add an abort on coroutine termination if there are
> still outstanding schedules, as that is preferable to operating in the
> weeds.

Sure, why not.  I'm all for adding more assertions (not less :)).

>> aio_co_schedule() on a running coroutine can only happen when the
>> coroutine is going to yield soon.
>>
> That is a bit vague.  What is "soon", and how does an external caller know
> if a coroutine is going to yield in this timeframe?

Soon really means "eventually"; basically if you do

        f();
        qemu_coroutine_yield();

then f() can call aio_co_wake() or aio_co_schedule() and knows that it
will be entirely thread-safe.

However, remember that only one aio_co_schedule() can be pending at a
single time; so if you have

        f();
        g();
        qemu_coroutine_yield();

either f() or g() can wake you up but not both.

Thanks,

Paolo