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

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Date:   Thu, 6 Jun 2019 02:43:40 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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

On Tue, Jun 04, 2019 at 09:41:04AM +0200, Geert Uytterhoeven wrote:
> 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.

But m68k is !SMP-only, correct?  If so, the only issues would be
interactions with interrupt handlers and the like, and doesn't current
m68k hardware use exact interrupts?  Or is it still possible to interrupt
an m68k in the middle of an instruction like it was in the bad old days?

							Thanx, Paul

> 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