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 11:56 AM, Daniel P. Berrangé wrote:
> > On Wed, Sep 11, 2019 at 05:42:00PM +0100, 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
> 
> spurious double space
> 
> >> g_auto infrastrcture (and thus whatever the compilers hooks are) to
> 
> infrastructure
> compiler's

Thanks.

Dave

> >> release it on all exits of the block.
> >>
> >> Note this macro has a variable declaration in, and hence is not in
> >> a while loop.
> >>
> 
> >> +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \
> >> +    _rcu_read_auto = 'x'; \
> >> +    rcu_read_lock();
> >> +
> > 
> > Functionally this works, but my gut feeling would be to follow
> > the design of GMutexLocker as-is:
> > 
> >   https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new
> > 
> > so you get a use pattern of
> > 
> >   g_autoptr(rcu_read_locker) locker = rcu_read_locker_new();
> 
> Another pattern to consider: nbdkit uses:
> 
> #define ACQUIRE_LOCK_FOR_CURRENT_SCOPE(mutex) \
>   CLEANUP_UNLOCK pthread_mutex_t *_lock = mutex; \
>   do { \
>     int _r = pthread_mutex_lock (_lock); \
>     assert (!_r); \
>   } while (0)
> 
> with later code calling:
> 
>   ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock);
> 
> > 
> > This makes it explicit that the code is creating a variable here, which
> > in turns means it is clear to force unlock early with
> > 
> >   g_clear_pointer(&locker, rcu_read_locker_free)
> 
> Yes, this aspect of glib is nicer than the corresponding nbdkit usage
> pattern.
> 
> -- 
> 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