Subject: Re: [PATCH v4 2/3] locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all archs

From: Will Deacon <[email-masked]>
Date: Wed, 20 Feb 2019 14:47:56 +0000

On Wed, Feb 13, 2019 at 05:00:16PM -0500, Waiman Long wrote:
> Currently, we have two different implementation of rwsem:
>  1) CONFIG_RWSEM_GENERIC_SPINLOCK (rwsem-spinlock.c)
>  2) CONFIG_RWSEM_XCHGADD_ALGORITHM (rwsem-xadd.c)
> 
> As we are going to use a single generic implementation for rwsem-xadd.c
> and no architecture-specific code will be needed, there is no point
> in keeping two different implementations of rwsem. In most cases, the
> performance of rwsem-spinlock.c will be worse. It also doesn't get all
> the performance tuning and optimizations that had been implemented in
> rwsem-xadd.c over the years.
> 
> For simplication, we are going to remove rwsem-spinlock.c and make all
> architectures use a single implementation of rwsem - rwsem-xadd.c.
> 
> All references to RWSEM_GENERIC_SPINLOCK and RWSEM_XCHGADD_ALGORITHM
> in the code are removed.
> 
> Suggested-by: Peter Zijlstra <[email-masked]>
> Signed-off-by: Waiman Long <[email-masked]>
> ---
>  arch/alpha/Kconfig              |   7 -
>  arch/arc/Kconfig                |   3 -
>  arch/arm/Kconfig                |   4 -
>  arch/arm64/Kconfig              |   3 -
>  arch/c6x/Kconfig                |   3 -
>  arch/csky/Kconfig               |   3 -
>  arch/h8300/Kconfig              |   3 -
>  arch/hexagon/Kconfig            |   6 -
>  arch/ia64/Kconfig               |   4 -
>  arch/m68k/Kconfig               |   7 -
>  arch/microblaze/Kconfig         |   6 -
>  arch/mips/Kconfig               |   7 -
>  arch/nds32/Kconfig              |   3 -
>  arch/nios2/Kconfig              |   3 -
>  arch/openrisc/Kconfig           |   6 -
>  arch/parisc/Kconfig             |   6 -
>  arch/powerpc/Kconfig            |   7 -
>  arch/riscv/Kconfig              |   3 -
>  arch/s390/Kconfig               |   6 -
>  arch/sh/Kconfig                 |   6 -
>  arch/sparc/Kconfig              |   8 -
>  arch/unicore32/Kconfig          |   6 -
>  arch/x86/Kconfig                |   3 -
>  arch/x86/um/Kconfig             |   6 -
>  arch/xtensa/Kconfig             |   3 -
>  include/linux/rwsem-spinlock.h  |  47 ------
>  include/linux/rwsem.h           |   5 -
>  kernel/Kconfig.locks            |   2 +-
>  kernel/locking/Makefile         |   4 +-
>  kernel/locking/rwsem-spinlock.c | 339 ----------------------------------------
>  kernel/locking/rwsem.h          |   3 -
>  31 files changed, 2 insertions(+), 520 deletions(-)
>  delete mode 100644 include/linux/rwsem-spinlock.h
>  delete mode 100644 kernel/locking/rwsem-spinlock.c

Another nice cleanup, and it looks like the optimistic spinning in the xadd
implementation is predicated on ARCH_SUPPORTS_ATOMIC_RMW, so we're all good
there too.

Acked-by: Will Deacon <[email-masked]>

Will