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

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Message-ID: <20190220144756.GK7523@fuggles.cambridge.arm.com>
Date:   Wed, 20 Feb 2019 14:47:56 +0000
From:   Will Deacon <will.deacon@....com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-c6x-dev@...ux-c6x.org,
        uclinux-h8-devel@...ts.sourceforge.jp,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        nios2-dev@...ts.rocketboards.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v4 2/3] locking/rwsem: Remove rwsem-spinlock.c & use
 rwsem-xadd.c for all archs

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 <peterz@...radead.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  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 <will.deacon@....com>

Will