Re: [Qemu-devel] [PATCH 1/3] rcu: Add automatically released rcu_read_lock variant

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eric Blake (eblake@redhat.com) wrote:
> On 9/11/19 11:42 AM, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > RCU_READ_LOCK_AUTO takes the rcu_read_lock  and then uses glib's
> > g_auto infrastrcture (and thus whatever the compilers hooks are) to
> > release it on all exits of the block.
> > 
> > Note this macro has a variable declaration in, and hence is not in
> > a while loop.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  include/qemu/rcu.h | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> > index 22876d1428..6a25b27d28 100644
> > --- a/include/qemu/rcu.h
> > +++ b/include/qemu/rcu.h
> > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func);
> >        }),                                                                \
> >        (RCUCBFunc *)g_free);
> >  
> > +typedef char rcu_read_auto_t;
> 
> Declaring new types ending in _t collides with the namespace reserved by
> POSIX.  While I don't think it will bite us, it's still worth
> considering if a different name is better.

Thanks, I've renamed it to 'RCUReadAuto' which is closer to what we
normally use for typedef's (albeit normally of structs)

Dave

> > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r)
> > +{
> > +  rcu_read_unlock();
> > +}
> > +
> > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock)
> > +
> > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \
> > +    _rcu_read_auto = 'x'; \
> 
> I'm a bit lost at where _rcu_read_auto is declared.  (I could understand
> if an earlier macro had created that typedef via concatenating _ with
> rcu_read_auto_t, but making the preprocessor drop _t is not possible. Is
> this a typo, and if so, why did the compiler not complain?)
> 
> > +    rcu_read_lock();
> > +
> >  #ifdef __cplusplus
> >  }
> >  #endif
> > 
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 



--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK