Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value

[Will Deacon]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> ---
>  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@xxxxxxx>

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

Will