linux-kernel - Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond

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Date:   Fri, 27 Mar 2020 00:37:35 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com
Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()

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