Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()

[Rasmus Villemoes]

On 24/03/2020 14.56, Peter Zijlstra wrote:
> Extend the static_call infrastructure to optimize the following common
> pattern:
> 
> 	if (func_ptr)
> 		func_ptr(args...)
> 

> +#define DEFINE_STATIC_COND_CALL(name, _func)				\
> +	DECLARE_STATIC_CALL(name, _func);				\
> +	struct static_call_key STATIC_CALL_NAME(name) = {		\
> +		.func = NULL,						\
> +	}
> +
>  #define static_call(name)						\
>  	((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_NAME(name).func))
>  
> +#define static_cond_call(name)						\
> +	if (STATIC_CALL_NAME(name).func)				\
> +		((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_NAME(name).func))
> +

What, apart from fear of being ridiculed by kernel folks, prevents the
compiler from reloading STATIC_CALL_NAME(name).func ? IOW, doesn't this
want a READ_ONCE somewhere?

And please remind me, what is the consensus for sizeof(long) loads: does
static_call() need load-tearing protection or not?

Rasmus