From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Yury Norov <yury.norov@gmail.com>,
linux-kernel@vger.kernel.org,
Alexey Klimov <klimov.linux@gmail.com>,
Andy Whitcroft <apw@canonical.com>,
Catalin Marinas <catalin.marinas@arm.com>,
David Laight <David.Laight@aculab.com>,
Dennis Zhou <dennis@kernel.org>,
Guenter Roeck <linux@roeck-us.net>,
Kees Cook <keescook@chromium.org>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Valentin Schneider <vschneid@redhat.com>,
Sven Schnelle <svens@linux.ibm.com>,
Russell King <linux@armlinux.org.uk>
Subject: Re: [PATCH v4 3/4] lib/find_bit: optimize find_next_bit() functions
Date: Tue, 20 Sep 2022 14:59:53 +0300 [thread overview]
Message-ID: <YymrOVsQW4YtoHve@smile.fi.intel.com> (raw)
In-Reply-To: <CAHk-=whELT9vVs+K1KqkySz+6LL21t7TqQXM_VWmrgGXancUHQ@mail.gmail.com>
On Mon, Sep 19, 2022 at 08:23:00AM -0700, Linus Torvalds wrote:
> On Mon, Sep 19, 2022 at 6:46 AM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
> >
> > > +#define FIND_NEXT_BIT(FETCH, MUNGE, size, start) \
> > > +({ \
> [..]
> > > +out: \
> >
> > I dunno if GCC expression limits the scope of goto labels
>
> No. Labels are function-global by default. If you need block-scope for
> them, you need to declare them explicitly in tha block before use with
> "__label__".
>
> > but on the safe side you can add a prefix to it, so it becomes:
> >
> > FIND_NEXT_BIT_out:
>
> That doesn't really help, since if you were to use the macro twice,
> you'd still get a name clash.
>
> That said, I'm not convinced any of this matters, since these macros
> aren't supposed to be used anywhere else, and in fact, they aren't
> even in any header file that would allow anybody else to use them.
>
> So I think all the normal "make macros safe" rules are simply
> irrelevant for these cases - despite the readable name, these macros
> are local special cases for code generation and avoiding duplication,
> not generic "use this macro to find a bit".
>
> So it's one thing if a macro is in a header file to be used by random
> code. It's a different thing entirely if it's a specialized local
> macro for a local issue, that nobody else is ever going to even see.
>
> So I don't think it would be wrong to use __label__ to block-scope it,
> or to use a longer name, but I also don't think it's really required.
>
> It's not exactly super-common, but we have various cases of macros
> that generate full (or partial) function bodies in various places,
> where the macro does various things that should *never* be done in a
> "regular" macro that gets used by normal code.
>
> You can see one class of this with something like
>
> git grep '^static.*##.*(.*\\$' -- '*.c'
>
> but to *really* go blind, see the "SYSCALL_DEFINE*()" macros in
> <linux/syscalls.h>.
>
> Those will mess with your mind, and go "whoever wrote those macros
> needs to be institutionalized". They do some impressive things,
> including creating local functions _and_ starting a new function
> definition where the actual code then isn't part of the macro, but
> actually just continues where the macro was used.
>
> Which is all very natural and actually looks quite nice and readable
> in the places that use it, with users looking like
>
> SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> {
> int fd;
> struct pid *p;
> ...
>
> which is all pretty legible. But there's no question that that macro
> violates every single "normal" macro rule.
>
> The FIND_NEXT_BIT() macro in comparison is pretty tame.
Thanks for elaboration. It makes a lot of sense and something TIL material
to me.
--
With Best Regards,
Andy Shevchenko
next prev parent reply other threads:[~2022-09-20 12:00 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-15 2:07 [PATCH v4 0/4] lib: optimize find_bit() functions Yury Norov
2022-09-15 2:07 ` [PATCH v4 1/4] lib/find_bit: introduce FIND_FIRST_BIT() macro Yury Norov
2022-09-15 2:07 ` [PATCH v4 2/4] lib/find_bit: create find_first_zero_bit_le() Yury Norov
2022-09-19 13:45 ` Andy Shevchenko
2022-09-15 2:07 ` [PATCH v4 3/4] lib/find_bit: optimize find_next_bit() functions Yury Norov
2022-09-15 16:25 ` Valentin Schneider
2022-09-19 13:45 ` Andy Shevchenko
2022-09-19 15:23 ` Linus Torvalds
2022-09-20 11:59 ` Andy Shevchenko [this message]
2022-09-20 1:41 ` Yury Norov
2022-09-15 2:07 ` [PATCH v4 4/4] tools: sync find_bit() implementation Yury Norov
2022-09-21 15:40 ` [PATCH v4 0/4] lib: optimize find_bit() functions Yury Norov
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