From: Boqun Feng <boqun.feng@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>, Ingo Molnar <mingo@kernel.org>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Documentation/atomic_t: Clarify signed vs unsigned
Date: Tue, 12 Feb 2019 09:03:55 +0800 [thread overview]
Message-ID: <20190212010355.GA11137@tardis> (raw)
In-Reply-To: <20190211170943.GP32477@hirez.programming.kicks-ass.net>
[-- Attachment #1: Type: text/plain, Size: 1464 bytes --]
On Mon, Feb 11, 2019 at 06:09:43PM +0100, Peter Zijlstra wrote:
>
> Clarify the whole signed vs unsigned issue for atomic_t.
>
> There has been enough confusion on this topic to warrant a few explicit
> words I feel.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
FWIW
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Regards,
Boqun
> ---
> Documentation/atomic_t.txt | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
> diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> index 913396ac5824..dca3fb0554db 100644
> --- a/Documentation/atomic_t.txt
> +++ b/Documentation/atomic_t.txt
> @@ -56,6 +56,23 @@ The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for
> smp_mb__{before,after}_atomic()
>
>
> +TYPES (signed vs unsigned)
> +-----
> +
> +While atomic_t, atomic_long_t and atomic64_t use int, long and s64
> +respectively (for hysterical raisins), the kernel uses -fno-strict-overflow
> +(which implies -fwrapv) and defines signed overflow to behave like
> +2s-complement.
> +
> +Therefore, an explicitly unsigned variant of the atomic ops is strictly
> +unnecessary and we can simply cast, there is no UB.
> +
> +There was a bug in UBSAN prior to GCC-8 that would generate UB warnings for
> +signed types.
> +
> +With this we also conform to the C/C++ _Atomic behaviour and things like
> +P1236R1.
> +
>
> SEMANTICS
> ---------
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]
next prev parent reply other threads:[~2019-02-12 0:55 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-11 17:09 [PATCH] Documentation/atomic_t: Clarify signed vs unsigned Peter Zijlstra
2019-02-11 17:28 ` Will Deacon
2019-02-11 21:13 ` Paul E. McKenney
2019-02-12 1:03 ` Boqun Feng [this message]
2019-02-12 1:29 ` Paul E. McKenney
2019-02-13 7:54 ` Ingo Molnar
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=20190212010355.GA11137@tardis \
--to=boqun.feng@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=paulmck@linux.vnet.ibm.com \
--cc=peterz@infradead.org \
--cc=will.deacon@arm.com \
/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 a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).