Re: [Qemu-devel] [RFC PATCH 2/3] sheepdog: pass NULL for io_flush

[Stefan Hajnoczi]

On Tue, Apr 02, 2013 at 10:37:24AM +0200, Kevin Wolf wrote:
> Am 28.03.2013 um 22:52 hat Anthony Liguori geschrieben:
> > Signed-off-by: Anthony Liguori <address@hidden>
> > ---
> >  block/sheepdog.c | 11 ++---------
> >  1 file changed, 2 insertions(+), 9 deletions(-)
> > 
> > diff --git a/block/sheepdog.c b/block/sheepdog.c
> > index bb67c4c..2bccd9b 100644
> > --- a/block/sheepdog.c
> > +++ b/block/sheepdog.c
> > @@ -503,13 +503,6 @@ static void restart_co_req(void *opaque)
> >      qemu_coroutine_enter(co, NULL);
> >  }
> >  
> > -static int have_co_req(void *opaque)
> > -{
> > -    /* this handler is set only when there is a pending request, so
> > -     * always returns 1. */
> 
> Now you return 1 even when no request is pending (which is the case in
> which no io_flush handler would be set before). Why is this correct?
> (This is actually a question about PATCH 1/3, I just noticed it here.
> Are there more cases like this?)

The trick with return 1 handlers is that they are deleted when there are
no more requests.  The pattern is:

...begin processing request...
qemu_set_fd_handler2(fd, ..., have_co_req);
qemu_coroutine_yield()
qemu_set_fd_handler2(fd, NULL, NULL, NULL, NULL); /* delete handler */
...finish processing request...

Stefan