Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock if possible

Jan Beulich
2011-03-24T08:51:43Z

>>> On 24.03.11 at 05:56, Nikanth Karthikesan <knikanth@suse.de> wrote:
> On x86_64 SMP with lots of CPU atomic instructions which assert the LOCK #
> signal can stall other CPUs. And as the number of cores increase this
> penalty
> scales proportionately. So it is best to try and avoid atomic instructions
> wherever possible. test_and_set_bit_lock() can avoid using LOCK_PREFIX if it
> finds the bit set already.

Why don't you do this in test_and_set_bit() instead?

Jan

> Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
>
> ---
>
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index 903683b..26a42ff 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -203,19 +203,6 @@ static inline int test_and_set_bit(int nr, volatile
> unsigned long *addr)
> }
>
> /**
> - * test_and_set_bit_lock - Set a bit and return its old value for lock
> - * @nr: Bit to set
> - * @addr: Address to count from
> - *
> - * This is the same as test_and_set_bit on x86.
> - */
> -static __always_inline int
> -test_and_set_bit_lock(int nr, volatile unsigned long *addr)
> -{
> - return test_and_set_bit(nr, addr);
> -}
> -
> -/**
> * __test_and_set_bit - Set a bit and return its old value
> * @nr: Bit to set
> * @addr: Address to count from
> @@ -339,6 +326,25 @@ static int test_bit(int nr, const volatile unsigned long
> *addr);
> : variable_test_bit((nr), (addr)))
>
> /**
> + * test_and_set_bit_lock - Set a bit and return its old value for lock
> + * @nr: Bit to set
> + * @addr: Address to count from
> + *
> + * This is the same as test_and_set_bit on x86. But atomic operation is
> + * avoided, if the bit was already set.
> + */
> +static __always_inline int
> +test_and_set_bit_lock(int nr, volatile unsigned long *addr)
> +{
> +#ifdef CONFIG_SMP
> + barrier();
> + if (test_bit(nr, addr))
> + return 1;
> +#endif
> + return test_and_set_bit(nr, addr);
> +}
> +
> +/**
> * __ffs - find first set bit in word
> * @word: The word to search
> *