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

[Kevin Wolf <kwolf@redhat.com>]

Am 16.01.2020 um 16:13 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 16.01.2020 um 10:45 hat Markus Armbruster geschrieben:
> >> Kevin Wolf <kwolf@redhat.com> writes:
> >> > block_resize is safe to run in a coroutine, so use it as an example for
> >> > the new 'coroutine': true annotation in the QAPI schema.
> >> >
> >> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> >> > Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> >
> >> > diff --git a/blockdev.c b/blockdev.c
> >> > index 8e029e9c01..b5e5d1e072 100644
> >> > --- a/blockdev.c
> >> > +++ b/blockdev.c
> >> > @@ -3161,9 +3161,9 @@ void hmp_drive_del(Monitor *mon, const QDict *qdict)
> >> >      aio_context_release(aio_context);
> >> >  }
> >> >  
> >> > -void qmp_block_resize(bool has_device, const char *device,
> >> > -                      bool has_node_name, const char *node_name,
> >> > -                      int64_t size, Error **errp)
> >> > +void coroutine_fn qmp_block_resize(bool has_device, const char *device,
> >> > +                                   bool has_node_name, const char *node_name,
> >> > +                                   int64_t size, Error **errp)
> >> >  {
> >> >      Error *local_err = NULL;
> >> >      BlockBackend *blk = NULL;
> >> 
> >> 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.

Kevin