Re: [RFC] change non-atomic bitops method

Uwe Kleine-König
2015-02-02T19:31:58Z

On Mon, Feb 02, 2015 at 11:55:03AM +0800, Wang, Yalin wrote:
> This patch change non-atomic bitops,
> add a if() condition to test it, before set/clear the bit.
> so that we don't need dirty the cache line, if this bit
> have been set or clear. On SMP system, dirty cache line will
> need invalidate other processors cache line, this will have
> some impact on SMP systems.
>
> Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
> ---
> include/asm-generic/bitops/non-atomic.h | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/include/asm-generic/bitops/non-atomic.h b/include/asm-generic/bitops/non-atomic.h
> index 697cc2b..e4ef18a 100644
> --- a/include/asm-generic/bitops/non-atomic.h
> +++ b/include/asm-generic/bitops/non-atomic.h
> @@ -17,7 +17,9 @@ static inline void __set_bit(int nr, volatile unsigned long *addr)
> unsigned long mask = BIT_MASK(nr);
> unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
>
> - *p |= mask;
> + if ((*p & mask) == 0)
> + *p |= mask;
Care to fix the double space here while touching the code?

I think the more natural check here is:

if ((~*p & mask) != 0)
*p |= mask;

Might be a matter of taste, but this check is equivalent to

*p != (*p | mask)

which is what you really want to test for. (Your check only has this
property for values of mask that have a single bit set, which is ok here
of course.)

> +
> }
>
> static inline void __clear_bit(int nr, volatile unsigned long *addr)
> @@ -25,7 +27,8 @@ static inline void __clear_bit(int nr, volatile unsigned long *addr)
> unsigned long mask = BIT_MASK(nr);
> unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
>
> - *p &= ~mask;
> + if ((*p & mask) != 0)
> + *p &= ~mask;
This is already fine.

> }
>
> /**
> @@ -60,7 +63,8 @@ static inline int __test_and_set_bit(int nr, volatile unsigned long *addr)
> unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> unsigned long old = *p;
>
> - *p = old | mask;
> + if ((old & mask) == 0)
> + *p = old | mask;
Here it would be:

if ((~old & mask) != 0)

> return (old & mask) != 0;
> }

Best regards
Uwe

--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |