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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Will Deacon <will@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Android Kernel Team <kernel-team@android.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading from scalar types
Date: Mon, 13 Jan 2020 11:31:56 -0800
Message-ID: <CAHk-=wirAWFOrfD4us1FepP0vWkZMpnqXusJyKHCqwBVsR43CA@mail.gmail.com> (raw)
In-Reply-To: <20200113145954.GB4458@willie-the-truck>

On Mon, Jan 13, 2020 at 7:00 AM Will Deacon <will@kernel.org> wrote:
>
> I can't disagree with that, but the only option we've come up with so far
> that solves this in the READ_ONCE() macro itself is the thing from PeterZ:
>
> // Insert big fat comment here
> #define unqual_typeof(x)    typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; }))

I'm with Luc on this - that not only looks gcc-specific, it looks
fragile too, in that it's not obvious that "_Atomic typeof(x)" really
is guaranteed to do what we want.

> So I suppose my question is: how ill does this code really make you feel?

I wish the code was more obvious.

One way to do that might be to do your approach, but just write it as
a series of macros that makes it a bit more understandable what it
does.

Maybe it's just because of a "pee in the snow" effect, but I think
this is easier to explain:

  #define __pick_scalar_type(x,type,otherwise)          \
        __builtin_choose_expr(__same_type(x,type), (type)0, otherwise)

  #define __pick_integer_type(x, type, otherwise)       \
        __pick_scalar_type(x, unsigned type,            \
          __pick_scalar_type(x, signed type, otherwise))

  #define __unqual_scalar_typeof(x) typeof(             \
        __pick_integer_type(x, char,                    \
          __pick_integer_type(x, short,                 \
            __pick_integer_type(x, int,                 \
              __pick_integer_type(x, long,              \
                __pick_integer_type(x, long long, x))))))

just because you there's less repeated noise, and the repetition there
is is simpler.

So still "Eww", but maybe not quite _as_ "Eww".

             Linus

  parent reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-10 16:56 [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen Will Deacon
2020-01-10 16:56 ` [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC prior to version 4.8 Will Deacon
2020-01-10 17:35   ` Arnd Bergmann
2020-01-10 17:53     ` Joe Perches
2020-01-13 14:39       ` Arnd Bergmann
2020-01-13 15:35         ` Masahiro Yamada
2020-01-13 14:27     ` Will Deacon
2020-01-14 21:39     ` Nick Desaulniers
2020-01-15 10:35       ` David Laight
2020-01-15 10:49       ` Arnd Bergmann
2020-01-10 16:56 ` [RFC PATCH 2/8] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-01-10 16:56 ` [RFC PATCH 3/8] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-01-10 16:56 ` [RFC PATCH 4/8] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-01-10 16:56 ` [RFC PATCH 5/8] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-01-10 19:24   ` Arnd Bergmann
2020-01-13 16:16     ` Will Deacon
2020-01-10 16:56 ` [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-01-10 18:54   ` Linus Torvalds
2020-01-13 14:59     ` Will Deacon
2020-01-13 17:42       ` Luc Van Oostenryck
2020-01-13 19:31       ` Linus Torvalds [this message]
2020-01-10 16:56 ` [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-01-10 19:42   ` Arnd Bergmann
2020-01-13 15:01     ` Will Deacon
2020-01-10 16:56 ` [RFC PATCH 8/8] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-01-10 17:45   ` Mark Rutland
2020-01-10 17:58 ` [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen Arnd Bergmann
2020-01-10 19:46 ` Arnd Bergmann
2020-01-10 20:14   ` Linus Torvalds
2020-01-13 13:03     ` Arnd Bergmann
2020-01-13 11:20 ` David Laight
2020-01-13 12:40 ` Christian Borntraeger

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