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

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Date:   Mon, 9 Jul 2018 14:07:39 +0100
From:   Will Deacon <will.deacon@....com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:     linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-arch@...r.kernel.org, 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>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] atomic{64}_t: Explicitly specify data storage length and
 alignment

On Mon, Jul 09, 2018 at 03:47:41PM +0300, Alexey Brodkin 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>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Darren Hart <dvhart@...radead.org>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: David Laight <David.Laight@...LAB.COM>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  arch/arm/include/asm/atomic.h                                 | 2 +-
>  include/asm-generic/atomic64.h                                | 2 +-
>  include/linux/types.h                                         | 4 ++--
>  tools/include/linux/types.h                                   | 2 +-
>  tools/testing/selftests/futex/include/atomic.h                | 2 +-
>  .../rcutorture/formal/srcu-cbmc/include/linux/types.h         | 4 ++--
>  6 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm/include/asm/atomic.h b/arch/arm/include/asm/atomic.h
> index 66d0e215a773..2ed6d7cf1407 100644
> --- a/arch/arm/include/asm/atomic.h
> +++ b/arch/arm/include/asm/atomic.h
> @@ -267,7 +267,7 @@ ATOMIC_OPS(xor, ^=, eor)
>  
>  #ifndef CONFIG_GENERIC_ATOMIC64
>  typedef struct {
> -	long long counter;
> +	u64 __aligned(8) counter;
>  } atomic64_t;

Long long is 8-byte aligned per EABI ARM, and we use the generic atomic64
infrastructure for OABI, so we don't need to change anything here afaict.

Will