linux-kernel - Re: single copy atomicity for double load/stores on 32-bit systems

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Date:   Tue, 4 Jun 2019 09:41:04 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <Will.Deacon@....com>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: single copy atomicity for double load/stores on 32-bit systems

Hi Paul,

On Mon, Jun 3, 2019 at 10:14 PM Paul E. McKenney <paulmck@...ux.ibm.com> wrote:
> On Mon, Jun 03, 2019 at 06:08:35PM +0000, Vineet Gupta wrote:
> > On 5/31/19 1:21 AM, Peter Zijlstra wrote:
> > >> I'm not sure how to interpret "natural alignment" for the case of double
> > >> load/stores on 32-bit systems where the hardware and ABI allow for 4 byte
> > >> alignment (ARCv2 LDD/STD, ARM LDRD/STRD ....)
> > > Natural alignment: !((uintptr_t)ptr % sizeof(*ptr))
> > >
> > > For any u64 type, that would give 8 byte alignment. the problem
> > > otherwise being that your data spans two lines/pages etc..
> >
> > Sure, but as Paul said, if the software doesn't expect them to be atomic by
> > default, they could span 2 hardware lines to keep the implementation simpler/sane.
>
> I could imagine 8-byte types being only four-byte aligned on 32-bit systems,
> but it would be quite a surprise on 64-bit systems.

Or two-byte aligned?

M68k started with a 16-bit data bus, and alignment rules were retained
when gaining a wider data bus.

BTW, do any platforms have issues with atomicity of 4-byte types on
16-bit data buses? I believe some embedded ARM or PowerPC do have
such buses.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds