Re: [Qemu-devel] [PATCH v2 1/5] rcu: Add automatically released rcu_read_lock variant

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/09/19 21:06, 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 infrastructure (and thus whatever the compiler's hooks are) to
> release it on all exits of the block.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  include/qemu/rcu.h | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> index 22876d1428..8768a7b60a 100644
> --- a/include/qemu/rcu.h
> +++ b/include/qemu/rcu.h
> @@ -154,6 +154,24 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func);
>        }),                                                                \
>        (RCUCBFunc *)g_free);
>  
> +typedef void RCUReadAuto;
> +static inline RCUReadAuto *rcu_read_auto_lock(void)
> +{
> +    rcu_read_lock();
> +    /* Anything non-NULL causes the cleanup function to be called */
> +    return (void *)0x1;

Doesn't this cause a warning (should be "(void *)(uintptr_t)1" instead)?

> +}
> +
> +static inline void rcu_read_auto_unlock(RCUReadAuto *r)
> +{
> +    rcu_read_unlock();
> +}
> +
> +G_DEFINE_AUTOPTR_CLEANUP_FUNC(RCUReadAuto, rcu_read_auto_unlock)
> +
> +#define RCU_READ_LOCK_AUTO \
> +    g_autoptr(RCUReadAuto) _rcu_read_auto = rcu_read_auto_lock()
> +
>  #ifdef __cplusplus
>  }
>  #endif
> 

Is it possible to make this a scope, like

	WITH_RCU_READ_LOCK() {
	}

?  Perhaps something like

#define WITH_RCU_READ_LOCK()  \
	WITH_RCU_READ_LOCK_(_rcu_read_auto##__COUNTER__)

#define WITH_RCU_READ_LOCK_(var) \
	for (g_autoptr(RCUReadAuto) var = rcu_read_auto_lock();
	     (var); rcu_read_auto_unlock(), (var) = NULL)

where the dummy variable doubles as an execute-once guard and the gauto
cleanup is still used in case of a "break".  I'm not sure if the above
raises a warning because of the variable declaration inside the for
loop, though.

Thanks,

Paolo