linux-kernel - Re: [PATCH v4 14/18] static_call: Add static_cond

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Date:   Tue, 5 May 2020 11:27:53 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, bristot@...hat.com,
        Jason Baron <jbaron@...mai.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v4 14/18] static_call: Add static_cond_call()

On Tue, May 5, 2020 at 11:13 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, May 5, 2020 at 2:36 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> >
> > HJ, Nick,
> >
> > Any chance any of you can see a way to make your respective compilers
> > not emit utter junk for this?
> >
> > On Mon, May 04, 2020 at 10:14:45PM +0200, Peter Zijlstra wrote:
> >
> > > https://godbolt.org/z/SDRG2q
>
> Woah, a godbolt link! Now we're speaking the same language.  What were
> you expecting? Us to remove the conditional check that a volatile read
> wasn't NULL? (Not using READ_ONCE, produces the direct tail call I
> suspect you're looking for, but am unsure if that's what you meant,
> and understand that's not a solution).  I am simultaneously impressed
> and disgusted by this btw, cool stuff.
>
> i.e.
> void *func = &name.func; \
> rather than
> void *func = READ_ONCE(name.func); \

Changing
void *func = READ_ONCE(name.func); \
to
void *func = &READ_ONCE(name.func); \
produces the tail call.  Not sure if that's relevant/what you were
looking for/even correct (haven't thought to hard about the
implications of that change; juggling other stuff ATM)

> (I'm surprised that `&name.func;` and `name.func;` also produce
> different results).
>
> > >
> > > ---
> > > #include <stddef.h>
> > >
> > >
> > > #define READ_ONCE(var)                (*((volatile typeof(var) *)&(var)))
> > > #define WRITE_ONCE(var, val)  (*((volatile typeof(var) *)&(var)) = (val))
> > >
> > > struct static_call_key {
> > >       void *func;
> > > };
> > >
> > > #define DECLARE_STATIC_CALL(name, func)       \
> > >       extern struct static_call_key name; \
> > >       extern typeof(func) __SCT__##name;
> > >
> > > #define DEFINE_STATIC_COND_CALL(name, _func) \
> > >       DECLARE_STATIC_CALL(name, _func) \
> > >       struct static_call_key name = { \
> > >               .func = NULL, \
> > >       }
> > >
> > > static void __static_call_nop(void)
> > > {
> > > }
> > >
> > > #define __static_cond_call(name) \
> > > ({ \
> > >       void *func = READ_ONCE(name.func); \
> > >       if (!func) \
> > >               func = &__static_call_nop; \
> > >       (typeof(__SCT__##name)*)func; \
> > > })
> > >
> > > #define static_cond_call(name) (void)__static_cond_call(name)
> > >
> > > static void inline static_call_update(struct static_call_key *call, void *func)
> > > {
> > >       WRITE_ONCE(call->func, func);
> > > }
> > >
> > > volatile int _x;
> > >
> > > void bar(int x)
> > > {
> > >       _x = x;
> > > }
> > >
> > > DEFINE_STATIC_COND_CALL(foo, bar);
> > >
> > > void ponies(int x)
> > > {
> > >       static_cond_call(foo)(x);
> > > }
>
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers