Re: [Qemu-devel] [RFC PATCH 1/3] balloon: Allow nested inhibits

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 18 Jul 2018 14:40:15 +0800
Peter Xu <peterx@redhat.com> wrote:

> On Tue, Jul 17, 2018 at 04:47:37PM -0600, Alex Williamson wrote:
> > A simple true/false internal state does not allow multiple users.  Fix
> > this within the existing interface by converting to a counter, so long
> > as the counter is elevated, ballooning is inhibited.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >  balloon.c |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/balloon.c b/balloon.c
> > index 6bf0a9681377..2a6a7e1a22a0 100644
> > --- a/balloon.c
> > +++ b/balloon.c
> > @@ -37,16 +37,17 @@
> >  static QEMUBalloonEvent *balloon_event_fn;
> >  static QEMUBalloonStatus *balloon_stat_fn;
> >  static void *balloon_opaque;
> > -static bool balloon_inhibited;
> > +static int balloon_inhibited;
> >  
> >  bool qemu_balloon_is_inhibited(void)
> >  {
> > -    return balloon_inhibited;
> > +    return balloon_inhibited > 0;
> >  }
> >  
> >  void qemu_balloon_inhibit(bool state)
> >  {
> > -    balloon_inhibited = state;
> > +    balloon_inhibited += (state ? 1 : -1);
> > +    assert(balloon_inhibited >= 0);  
> 
> Better do it atomically?

I'd assumed we're protected by the BQL anywhere this is called.  Is
that not the case?  Generally when I try to add any sort of locking to
QEMU it's shot down because the code paths are already serialized.
Thanks,

Alex