linux-kernel - Re: [PATCH] static_call: fix function type mismatch

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Date:   Thu, 25 Mar 2021 08:42:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] static_call: fix function type mismatch

On Thu, Mar 25, 2021 at 01:42:41AM +0100, Rasmus Villemoes wrote:
> > Actually, it looks like I can't select PREEMPT_DYNAMIC> and tweaking Kconfig
> 
> Ah, there's no prompt on the "bool" line, so it doesn't show up. That
> seems to be a mistake, since there's an elaborate help text which says
> 
>           The runtime overhead is negligible with
> HAVE_STATIC_CALL_INLINE enabled
>           but if runtime patching is not available for the specific
> architecture
>           then the potential overhead should be considered.
> 
> So it seems that it was meant to be "you can enable this if you really
> want".
> 
> to force enable it on arm64 results in a build error

Right, PREEMPT_DYNAMIC really hard relies on HAVE_STATIC_CALL

There's an implicit dependency in the select:

config PREEMPT
	...
	select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC

> > ("implicit declaration of function 'static_call_mod'").
> 
> Seems to be an omission in the last !HAVE_STATIC_CALL branch in
> static_call_types.h, and there's also no
> EXPORT_STATIC_CALL_TRAMP{,_GPL} in static_call.h for that case.

That interface doesn't make sense for !HAVE_STATIC_CALL. It's impossible
to not export the function pointer itself but still call it for
!HAVE_STATIC_CALL.