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

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Message-ID: <CAMuHMdWQNZ-Dpakqk7mTJ-O_H0QnCd-KUWpWNJC_d6Yw8GH0Kg@mail.gmail.com>
Date:   Mon, 9 Jul 2018 15:49:14 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        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>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] atomic{64}_t: Explicitly specify data storage length and alignment

On Mon, Jul 9, 2018 at 3:29 PM David Laight <David.Laight@...lab.com> wrote:
> From: Alexey Brodkin
> > Sent: 09 July 2018 13:48
> > 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.
> ...
> > diff --git a/include/asm-generic/atomic64.h b/include/asm-generic/atomic64.h
> > index 8d28eb010d0d..b94b749b5952 100644
> > --- 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;
> >  } atomic64_t;
>
> Apart from the fact that this changes the value from signed to unsigned
> should most of the architectures be using this generic definition?

64-bit architectures use the one from include/linux/types.h instead.

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