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

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Date:   Mon, 9 Jul 2018 15:35:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:     linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-arch@...r.kernel.org, Will Deacon <will.deacon@....com>,
        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:
> 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;

The type is wrong, atomic is signed, the alignment also really doesn't
matter, generic atomic64 is utter crap.

>  #define ATOMIC64_INIT(i)	{ (i) }
> diff --git a/include/linux/types.h b/include/linux/types.h
> index 9834e90aa010..e2f631782621 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -174,12 +174,12 @@ typedef phys_addr_t resource_size_t;
>  typedef unsigned long irq_hw_number_t;
>  
>  typedef struct {
> -	int counter;
> +	u32 __aligned(4) counter;
>  } atomic_t;

u32 is wrong, the atomic type is signed.

Also, if an architecture doesn't properly align its native machine word
size but requires alignment for atomics it's a broken architecture.

>  
>  #ifdef CONFIG_64BIT
>  typedef struct {
> -	long counter;
> +	u64 __aligned(8) counter;
>  } atomic64_t;
>  #endif
>  

Similar for this one, on 64bit archs that support atomics the native
64bit types (long included) had better already imply this alignment.