linux-kernel - Re: [PATCH 3/3] bitops.h: set_mask

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Message-ID: <20190114190025.GA29167@fuggles.cambridge.arm.com>
Date:   Mon, 14 Jan 2019 19:00:25 +0000
From:   Will Deacon <will.deacon@....com>
To:     Vineet Gupta <vineet.gupta1@...opsys.com>
Cc:     linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-mm@...ck.org, peterz@...radead.org,
        Miklos Szeredi <mszeredi@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jani Nikula <jani.nikula@...el.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value

On Thu, Jan 10, 2019 at 04:26:27PM -0800, Vineet Gupta wrote:
> | > Also, set_mask_bits is used in fs quite a bit and we can possibly come up
> | > with a generic llsc based implementation (w/o the cmpxchg loop)
> |
> | May I also suggest changing the return value of set_mask_bits() to old.
> |
> | You can compute the new value given old, but you cannot compute the old
> | value given new, therefore old is the better return value. Also, no
> | current user seems to use the return value, so changing it is without
> | risk.
> 
> Link: http://lkml.kernel.org/g/20150807110955.GH16853@twins.programming.kicks-ass.net
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Cc: Miklos Szeredi <mszeredi@...hat.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Jani Nikula <jani.nikula@...el.com>
> Cc: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Will Deacon <will.deacon@....com>
> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> ---
>  include/linux/bitops.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index 705f7c442691..602af23b98c7 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -246,7 +246,7 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr,
>  		new__ = (old__ & ~mask__) | bits__;		\
>  	} while (cmpxchg(ptr, old__, new__) != old__);		\
>  								\
> -	new__;							\
> +	old__;							\
>  })
>  #endif

Acked-by: Will Deacon <will.deacon@....com>

May also explain why no in-tree users appear to use the return value!

Will