linux-kernel - Re: [PATCH 10/15] static_call: Add basic static call infrastructure

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Date:   Fri, 7 Jun 2019 10:28:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 10/15] static_call: Add basic static call infrastructure

On Thu, Jun 06, 2019 at 10:44:23PM +0000, Nadav Amit wrote:
> > + * Usage example:
> > + *
> > + *   # Start with the following functions (with identical prototypes):
> > + *   int func_a(int arg1, int arg2);
> > + *   int func_b(int arg1, int arg2);
> > + *
> > + *   # Define a 'my_key' reference, associated with func_a() by default
> > + *   DEFINE_STATIC_CALL(my_key, func_a);
> > + *
> > + *   # Call func_a()
> > + *   static_call(my_key, arg1, arg2);
> > + *
> > + *   # Update 'my_key' to point to func_b()
> > + *   static_call_update(my_key, func_b);
> > + *
> > + *   # Call func_b()
> > + *   static_call(my_key, arg1, arg2);
> 
> I think that this calling interface is not very intuitive.

Yeah, it is somewhat unfortunate..

> I understand that
> the macros/objtool cannot allow the calling interface to be completely
> transparent (as compiler plugin could). But, can the macros be used to
> paste the key with the “static_call”? I think that having something like:
> 
>   static_call__func(arg1, arg2)
> 
> Is more readable than
> 
>   static_call(func, arg1, arg2)

Doesn't really make it much better for me; I think I'd prefer to switch
to the GCC plugin scheme over this.  ISTR there being some propotypes
there, but I couldn't quickly locate them.

> > +}
> > +
> > +#define static_call_update(key, func)					\
> > +({									\
> > +	BUILD_BUG_ON(!__same_type(func, STATIC_CALL_TRAMP(key)));	\
> > +	__static_call_update(&key, func);				\
> > +})
> 
> Is this safe against concurrent module removal?

It is for CONFIG_MODULE=n :-)