linux-kernel - Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading from scalar types

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Date:   Wed, 22 Apr 2020 14:11:29 +0100
From:   Will Deacon <will@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...roid.com, Mark Rutland <mark.rutland@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading
 from scalar types

On Wed, Apr 22, 2020 at 06:48:07AM -0500, Segher Boessenkool wrote:
> On Wed, Apr 22, 2020 at 12:25:03PM +0200, Rasmus Villemoes wrote:
> > On 21/04/2020 17.15, Will Deacon wrote:
> > > Unfortunately, dropping pointer qualifiers inside the macro poses quite
> > > a challenge, especially since the pointed-to type is permitted to be an
> > > aggregate, and this is relied upon by mm/ code accessing things like
> > > 'pmd_t'. Based on numerous hacks and discussions on the mailing list,
> > > this is the best I've managed to come up with.
> > 
> > Hm, maybe this can be brought to work, only very lightly tested. It
> > basically abuses what -Wignored-qualifiers points out:
> > 
> >   warning: type qualifiers ignored on function return type
> > 
> > Example showing the idea:
> > 
> > const int c(void);
> > volatile int v(void);
> > 
> > int hack(int x, int y)
> > {
> > 	typeof(c()) a = x;
> > 	typeof(v()) b = y;
> > 
> > 	a += b;
> > 	b += a;
> > 	a += b;
> > 	return a;
> > }
> 
> Nasty.  I like it :-)
> 
> > Since that compiles, a cannot be const-qualified, and the generated code
> > certainly suggests that b is not volatile-qualified. So something like
> > 
> > #define unqual_type(x) _unqual_type(x, unique_id_dance)
> > #define _unqual_type(x, id) typeof( ({
> >   typeof(x) id(void);
> >   id();
> > }) )
> > 
> > and perhaps some _Pragma("GCC diagnostic push")/_Pragma("GCC diagnostic
> > ignored -Wignored-qualifiers")/_Pragma("GCC diagnostic pop") could
> > prevent the warning (which is in -Wextra, so I don't think it would
> > appear in a normal build anyway).
> > 
> > No idea how well any of this would work across gcc versions or with clang.
> 
> https://gcc.gnu.org/legacy-ml/gcc-patches/2016-05/msg01054.html
> 
> This is defined to work this way in ISO C since C11.
> 
> But, it doesn't work with GCC before GCC 7 :-(

Damn, that's quite a cool hack! Maybe we'll be able to implement it in a
few years time ;)

WIll