linux-kernel - Re: [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading from scalar types

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Date:   Mon, 13 Jan 2020 18:42:25 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Will Deacon <will@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading
 from scalar types

On Mon, Jan 13, 2020 at 02:59:54PM +0000, Will Deacon wrote:
> // Insert big fat comment here
> #define unqual_typeof(x)    typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; }))
> 
> That apparently *requires* GCC 4.8, but I think the question is more about
> whether it's easier to stomach the funny use of _Atomic or the nested
> __builtin_choose_expr() I have here. I'm also worried about how reliable
> the _Atomic thing is, or whether it's just an artifact of how GCC happens
> to work today.

As far as I understand it, it's an artifact of how GCC works today (it
was added to support the type-generic macros in <tgmath.h>).
I also think it's also quite fragile, for example, the unqualified type
is returned if typeof's argument is an expression but not if it's a
'typename'. IOW:
	typeof(_Atomic typeof(const int))
returns 'const int', while
	typeof(({_Atomic typeof(const int) x; x; }))
returns 'int'.

-- Luc