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Subject: Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()
To:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, rostedt@goodmis.org,
        mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com,
        torvalds@linux-foundation.org, tglx@linutronix.de,
        mingo@kernel.org, namit@vmware.com, hpa@zytor.com, luto@kernel.org,
        ard.biesheuvel@linaro.org, jpoimboe@redhat.com
References: <20200324135603.483964896@infradead.org>
 <20200324142246.127013582@infradead.org>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <10ef25bf-87df-6917-1d50-c29ece442766@rasmusvillemoes.dk>
Date:   Fri, 27 Mar 2020 00:37:35 +0100
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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