linux-kernel - Re: [tip:x86/fpu] bitops: Add clear/set

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Message-Id: <20171105155334.GA3225@osiris>
Date:   Sun, 5 Nov 2017 16:53:34 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     hpa@...or.com, peterz@...radead.org, mingo@...nel.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, ak@...ux.intel.com
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/fpu] bitops: Add clear/set_bit32() to linux/bitops.h

On Tue, Oct 17, 2017 at 09:21:46AM -0700, tip-bot for Andi Kleen wrote:
> Commit-ID:  cbe96375025e14fc76f9ed42ee5225120d7210f8
> Gitweb:     https://git.kernel.org/tip/cbe96375025e14fc76f9ed42ee5225120d7210f8
> Author:     Andi Kleen <ak@...ux.intel.com>
> AuthorDate: Fri, 13 Oct 2017 14:56:41 -0700
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Tue, 17 Oct 2017 17:14:56 +0200
> 
> bitops: Add clear/set_bit32() to linux/bitops.h
> 
> Add two simple wrappers around set_bit/clear_bit() that accept
> the common case of an u32 array. This avoids writing
> casts in all callers.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Link: http://lkml.kernel.org/r/20171013215645.23166-2-andi@firstfloor.org
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  include/linux/bitops.h | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)

...

> + * set_bit32 - Set a bit in memory for u32 array
> + * @nr: Bit to clear
> + * @addr: u32 * address of bitmap
> + *
> + * Same as set_bit, but avoids needing casts for u32 arrays.
> + */
> +
> +static __always_inline void set_bit32(long nr, volatile u32 *addr)
> +{
> +	set_bit(nr, (volatile unsigned long *)addr);
> +}

This does not work at all on 64 bit big endian machines. If e.g. the array
would contain only one 32 bit member set_bit() would write to whatever is
behind the array.