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

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Message-ID: <20200327100831.GT20713@hirez.programming.kicks-ass.net>
Date:   Fri, 27 Mar 2020 11:08:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     x86@...nel.org, 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 Fri, Mar 27, 2020 at 12:37:35AM +0100, Rasmus Villemoes wrote:
> 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?

Hurmph.. I suspect you're quite right, but at the same time I can't seem
to write a macro that does that :/ Let me try harder.

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

We all like to believe compilers are broken when they tear naturally
aligned words, but we're also not quite comfortable trusting that.