linux-kernel - Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING

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Date:	Wed, 15 Jun 2016 19:45:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS
 for better disambiguation

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.