Re: [PATCH 2/9] block-copy: add missing coroutine_fn annotations

[Emanuele Giuseppe Esposito]

Am 03/11/2022 um 19:36 schrieb Paolo Bonzini:
> On 11/3/22 19:06, Kevin Wolf wrote:
>> I think it can make sense to have coroutine_fn as a documentation for
>> things that are only ever called in a coroutine even if they could
>> theoretically also work outside of coroutine context.
>>
>> Otherwise, when we want to introduce a coroutine_fn call somewhere, it's
>> not only less obvious that it's even possible to do, but we'll have to
>> add potentially many additional coroutine_fn annotations in the whole
>> call chain in an otherwise unrelated patch.
> 
> This is true.  On the other hand, coroutine_fn also means "this is
> allowed to suspend", which may have implications on the need for locking
> in the caller.  So you need to judge case-by-case.
> 
> If there are good reasons to add the note, you could add an assertion
> that you are qemu_in_coroutine(), which notes that you are in a
> coroutine but you don't suspend.  In this case however I don't think
> it's likely that there will be a coroutine_fn call added later.
> 
> I guess I tend to err on the side of "it's good that it's not obvious
> that you can call a coroutine_fn", but I also need to correct myself as
> qemu_coroutine_yield() is not the only leaf; there is also
> qemu_co_queue_next() and qemu_co_queue_restart_all(), which are
> coroutine_fn because they rely on the queuing behavior of
> aio_co_enter().  In this case I violated my own rule because it is
> always a bug to call these functions outside coroutine context.
> 

But isn't it a bug also not to mark a function _only_ called by
coroutine_fn? My point is that if this function is an implementation of
a BlockDriver callback marked as coroutine_fn (like in patch 6 with
vmdk), then it would make sense.

This is actually the point of this serie (which I might not have
explained well in the cover letter), every function marked here is
eventually called by/calling a BlockDriver callback marked as coroutine_fn.

Currently we have something like this:
BlockDriver {
    void coroutine_fn (*bdrv_A)(void) = implA;
}

void coroutine_fn implA() {
    funcB();
    funcC();
}

void funcB() {}; <--- missing coroutine_fn?
void funcC() {}; <--- missing coroutine_fn?

In addition, as I understand draining is not allowed in coroutines. If a
function/callback only running in coroutines is not marked as
coroutine_fn, then it will be less obvious to notice that draining is
not allowed there.

Emanuele