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

[Eric Blake]

On 9/11/19 11:42 AM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> 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 <address@hidden>
> ---
>  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.

> +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

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