Re: [PATCH v2 2/9] block-copy: add missing coroutine_fn annotations

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

To sum up on what was discussed in this serie, I don't really see any
strong objection against these patches, so I will soon send v3 which is
pretty much the same except for patch 1, which will be removed.

I think these patches are useful and will be even more meaningful to the
reviewer when in the next few days I send all the rwlock patches.

What has been discussed so far (using QEMU_IN_COROUTINE, using some sort
of tool to automate everything, etc.) has been noted and as I understand
will be researched by Alberto.

Thank you,
Emanuele

Am 10/11/2022 um 11:52 schrieb Paolo Bonzini:
> On Wed, Nov 9, 2022 at 1:24 PM Emanuele Giuseppe Esposito
> <eesposit@redhat.com> wrote:
>>>> What I do know is that it's extremely confusing to understand if a
>>>> function that is *not* marked as coroutine_fn is actually being called
>>>> also from coroutines or not.
> 
> Agreed. This is a huge point in favor of pushing coroutine wrappers as
> far up in the call stack as possible, because it means more
> coroutine_fns and fewer mixed functions.
> 
>>> This is a lot better than our "coroutine_fn" sign, which actually do no
>>> check (and can't do). Don't you plan to swap a "coroutine_fn" noop
>>> marker with more meaningful IN_COROUTINE(); (or something like this,
>>> which just do assert(qemu_in_coroutine())) at start of the function? It
>>> would be a lot safer.
>>
>> CCing also Alberto and Paolo
>>
>> So basically I think what we need is something that scans the whole
>> block layer code and puts the right coroutine_fn annotations (or
>> assertions, if you want) in the right places.
> 
> coroutine_fn markers are done by Alberto's static analyzer, which I
> used to add coroutine_fn pretty much everywhere in the code base where
> they are *needed*. My rules are simple:
> 
> * there MUST be no calls from non-coroutine_fn to coroutine_fn, this is obvious
> 
> * there MUST be no blocking in coroutine_fn
> 
> * there SHOULD be no calls from coroutine_fn to generated_co_wrapper;
> use the wrapped *_co_* function directly instead.
> 
> To catch the last one, or possibly the last two, Alberto added
> no_coroutine_fn. In a perfect world non-marked functions would be
> "valid either in coroutine or non-coroutine function": they would call
> neither coroutine_fns nor no_coroutine_fns.
> 
> This is unfortunately easier said than done, but in order to move
> towards that case, I think we can look again at vrc and extend it with
> new commands. Alberto's work covers *local* tests, looking at one
> caller and one callee at a time. With vrc's knowledge of the global
> call graph, we can find *all* paths from a coroutine_fn to a
> generated_co_wrapper, including those that go through unmarked
> functions. Then there are two cases:
> 
> * if the unmarked function is never called from outside a coroutine,
> call the wrapped function and change it to coroutine_fn
> 
> * if the unmarked function can be called from outside a coroutine,
> change it to a coroutine_fn (renaming it) and add a
> generated_co_wrapper. Rinse and repeat.
> 
>> However, it would be nice to assign this to someone and do this
>> automatically, not doing it by hand. I am not sure if Alberto static
>> analyzer is currently capable of doing that.
> 
> I think the first step has to be done by hand, because it entails
> creating new generated_co_wrappers. Checking for regressions can then
> be done automatically though.
> 
> Paolo
>