Re: [Qemu-devel] [PATCH RFC 0/9] block: Rewrite block drain begin/end

[Kevin Wolf]

Am 01.12.2017 um 10:51 hat Fam Zheng geschrieben:
> On Thu, 11/30 17:04, Paolo Bonzini wrote:
> > On 30/11/2017 16:10, Kevin Wolf wrote:
> > >> Yes, I agree, but that (using CoMutex around graph change) requires
> > >> everything, especially the defer_to_main_loop_bh, runs in a coroutine
> > >> context, which is exactly what I mean by "introducing 'ubiquitous
> > >> coroutines'", because currently we don't have them.
> > > Is it hard to do, though? Instead of using a BH to switch to the main
> > > loop and outside of coroutine context, you could use aio_co_schedule()
> > > and yield, which would leave you in the main loop, but still in
> > > coroutine context.
> > 
> > Not that I think of, but just aio_co_schedule wouldn't work, because
> > "the coroutine must have yielded unless ctx is the context in which the
> > coroutine is running (i.e. the value of qemu_get_current_aio_context()
> > from the coroutine itself)".
> > 
> > So you'd have to use a bottom half that calls aio_co_schedule.  But that
> > would work.
> 
> We have QMP commands that can manupulate the graph which are all not 
> coroutines.
> I think running QMP commands in coroutines has it merit especially regarding 
> to
> the nested event loops.
> 
> Also the bdrv_close_all() and similar at the end of main() do draining too,
> which I'm not sure how to deal with. Maybe special case them and forget the
> draining CoMutex?

All of these cases have in common that they are the outermost layer with
respect to the block subsystem. This means that a nested event loop
there should be harmless because the callbacks called by it won't
influence callers further up in the call stack.

Kevin