linux-kernel - Re: [PATCH] atomic{64}_t: Explicitly specify data storage length and alignment

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Date:   Mon, 9 Jul 2018 14:55:38 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        Shuah Khan <shuah@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] atomic{64}_t: Explicitly specify data storage length and alignment

Hi Alexey,

On Mon, Jul 9, 2018 at 2:48 PM Alexey Brodkin
<Alexey.Brodkin@...opsys.com> wrote:
> Atomic instructions require data they operate on to be aligned
> according to data size. I.e. 32-bit atomic values must be 32-bit
> aligned while 64-bit values must be 64-bit aligned.
>
> Otherwise even if CPU may handle not-aligend normal data access,
> still atomic instructions fail and typically raise an exception
> leaving us dead in the water.
>
> This came-up during lengthly discussion here:
> http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004022.html
>
> Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>

Thanks for your patch!

> --- a/include/asm-generic/atomic64.h
> +++ b/include/asm-generic/atomic64.h
> @@ -13,7 +13,7 @@
>  #define _ASM_GENERIC_ATOMIC64_H
>
>  typedef struct {
> -       long long counter;
> +       u64 __aligned(8) counter;

I think you can drop the change to this file, as this is the generic
implementation using spinlocks, for processors that don't have 64-bit atomic
instructions.

>  } atomic64_t;

Reviewed-by: Geert Uytterhoeven <geert@...ux-m68k.org>

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