All of lore.kernel.org
 help / color / mirror / Atom feed
From: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
To: Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	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>,
	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 18:42:25 +0100	[thread overview]
Message-ID: <20200113174225.xs3n7t3obysbsmzd@ltop.local> (raw)
In-Reply-To: <20200113145954.GB4458@willie-the-truck>

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

  reply	other threads:[~2020-01-13 17:42 UTC|newest]

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 [this message]
2020-01-13 19:31       ` Linus Torvalds
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200113174225.xs3n7t3obysbsmzd@ltop.local \
    --to=luc.vanoostenryck@gmail.com \
    --cc=arnd@arndb.de \
    --cc=borntraeger@de.ibm.com \
    --cc=kernel-team@android.com \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mpe@ellerman.id.au \
    --cc=peterz@infradead.org \
    --cc=segher@kernel.crashing.org \
    --cc=torvalds@linux-foundation.org \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.