Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation

Peter Zijlstra
2016-06-15T17:45:08Z

On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote:
> +++ b/arch/alpha/include/asm/rwsem.h
> @@ -17,9 +17,9 @@
> #define RWSEM_UNLOCKED_VALUE 0x0000000000000000L
> #define RWSEM_ACTIVE_BIAS 0x0000000000000001L
> #define RWSEM_ACTIVE_MASK 0x00000000ffffffffL
> -#define RWSEM_WAITING_BIAS (-0x0000000100000000L)
> +#define RWSEM_WAITING_BIAS 0xc000000000000000L
> #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
> -#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +#define RWSEM_ACTIVE_WRITE_BIAS (-RWSEM_ACTIVE_MASK)

> +++ b/arch/ia64/include/asm/rwsem.h
> @@ -30,9 +30,9 @@
> #define RWSEM_UNLOCKED_VALUE __IA64_UL_CONST(0x0000000000000000)
> #define RWSEM_ACTIVE_BIAS (1L)
> #define RWSEM_ACTIVE_MASK (0xffffffffL)
> -#define RWSEM_WAITING_BIAS (-0x100000000L)
> +#define RWSEM_WAITING_BIAS (-(1L << 62))
> #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
> -#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +#define RWSEM_ACTIVE_WRITE_BIAS (-RWSEM_ACTIVE_MASK)

> +++ b/arch/s390/include/asm/rwsem.h
> @@ -42,9 +42,9 @@
> #define RWSEM_UNLOCKED_VALUE 0x0000000000000000L
> #define RWSEM_ACTIVE_BIAS 0x0000000000000001L
> #define RWSEM_ACTIVE_MASK 0x00000000ffffffffL
> -#define RWSEM_WAITING_BIAS (-0x0000000100000000L)
> +#define RWSEM_WAITING_BIAS 0xc000000000000000L
> #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
> -#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +#define RWSEM_ACTIVE_WRITE_BIAS (-RWSEM_ACTIVE_MASK)

> +++ b/arch/x86/include/asm/rwsem.h
> @@ -41,21 +41,23 @@
>
> /*
> * The bias values and the counter type limits the number of
> - * potential readers/writers to 32767 for 32 bits and 2147483647
> - * for 64 bits.
> + * potential writers to 16383 for 32 bits and 1073741823 for 64 bits.
> + * The combined readers and writers can go up to 65534 for 32-bits and
> + * 4294967294 for 64-bits.
> */
>
> #ifdef CONFIG_X86_64
> # define RWSEM_ACTIVE_MASK 0xffffffffL
> +# define RWSEM_WAITING_BIAS (-(1L << 62))
> #else
> # define RWSEM_ACTIVE_MASK 0x0000ffffL
> +# define RWSEM_WAITING_BIAS (-(1L << 30))
> #endif
>
> #define RWSEM_UNLOCKED_VALUE 0x00000000L
> #define RWSEM_ACTIVE_BIAS 0x00000001L
> -#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
> #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
> -#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +#define RWSEM_ACTIVE_WRITE_BIAS (-RWSEM_ACTIVE_MASK)

> +++ b/include/asm-generic/rwsem.h
> @@ -18,15 +18,16 @@
> */
> #ifdef CONFIG_64BIT
> # define RWSEM_ACTIVE_MASK 0xffffffffL
> +# define RWSEM_WAITING_BIAS (-(1L << 62))
> #else
> # define RWSEM_ACTIVE_MASK 0x0000ffffL
> +# define RWSEM_WAITING_BIAS (-(1L << 30))
> #endif
>
> #define RWSEM_UNLOCKED_VALUE 0x00000000L
> #define RWSEM_ACTIVE_BIAS 0x00000001L
> -#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
> #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
> -#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +#define RWSEM_ACTIVE_WRITE_BIAS (-RWSEM_ACTIVE_MASK)

Can't we collapse all that? They all seem very similar.