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

["Daniel P. Berrangé" <berrange@redhat.com>]

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
> 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;
> +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'; \
> +    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();

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)


Regards,
Daniel
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