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

[Waiman Long <waiman.long@hpe.com>]

On 06/15/2016 01:45 PM, Peter Zijlstra wrote:
> 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.

Yes, they are actually the same. I think we could extract the macro 
definitions into asm-generic/rwsem-macros.h, for instance, and make the 
architecture specific header file include the new generic header.

Cheers,
Longman