Re: [PATCH v3 4/4] block: Mark 'block_resize' as coroutine

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 17.01.2020 um 06:44 hat Markus Armbruster geschrieben:
>> Kevin Wolf <address@hidden> writes:
>> 
>> > Am 16.01.2020 um 16:13 hat Markus Armbruster geschrieben:
>> >> Kevin Wolf <address@hidden> writes:
>> >> 
>> >> > Am 16.01.2020 um 10:45 hat Markus Armbruster geschrieben:
[...]
>> >> >> Pardon my ignorant question: what exactly makes a function a
>> >> >> coroutine_fn?
>> >> >
>> >> > When Stefan requested adding the coroutine_fn marker, it seemed to make
>> >> > sense to me because the QMP dispatcher will always call it from
>> >> > coroutine context now, and being always run in coroutine context makes a
>> >> > function a coroutine_fn.
>> >> >
>> >> > However, it's also called from hmp_block_resize(), so at least for now
>> >> > coroutine_fn is actually wrong.
>> >> 
>> >> This answers the question when we mark a function a coroutine_fn.  I
>> >> meant to ask what conditions the function itself must satisfy to be
>> >> eligible for this mark.
>> >
>> > The requirement is actually not about the function itself, it's about
>> > the callers, as stated above.
>> >
>> > But being a coroutine_fn allows the function to call other functions
>> > that only work in coroutine context (other coroutine_fns). In the end
>> > the reason why a function only works in coroutine context is usually
>> > that it (or any other coroutine_fns called by it) could yield, which
>> > obviously doesn't work outside of coroutine contest.
>> 
>> Thanks.
>> 
>> I think "being always run in coroutine context makes a function a
>> coroutine_fn" is inaccurate.  It's "calling a coroutine_fn without
>> switching to coroutine context first when not already in coroutine
>> context".  The induction terminates at basic coroutine_fn like
>> qemu_coroutine_yield().
>
> I think we would tend to mark things as coroutine_fn even if they don't
> call other coroutine_fns (yet), but would be allowed to. But this is now
> really splitting hairs.

Your hair-splitting is my education :)

> Maybe I should just have referred to the documentation in the source:
>
> /**
>  * Mark a function that executes in coroutine context
>  *
>  * Functions that execute in coroutine context cannot be called directly from
>  * normal functions.  In the future it would be nice to enable compiler or
>  * static checker support for catching such errors.  This annotation might 
> make
>  * it possible and in the meantime it serves as documentation.
>  *
>  * For example:
>  *
>  *   static void coroutine_fn foo(void) {
>  *       ....
>  *   }
>  */
> #define coroutine_fn

I had read that, of course, but it didn't quite enlighten me, so I
asked.

Perhaps it would have if it said "Mark a function that expects to run in
coroutine context".