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    Subject[PATCH] mutex: Introduce arch_mutex_cpu_relax()
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    From: Gerald Schaefer <gerald.schaefer@de.ibm.com>

    The spinning mutex implementation uses cpu_relax() in busy loops as a
    compiler barrier. Depending on the architecture, cpu_relax() may do more
    than needed in this specific mutex spin loops. On System z we also give
    up the time slice of the virtual cpu in cpu_relax(), which prevents
    effective spinning on the mutex.

    This patch replaces cpu_relax() in the spinning mutex code with
    arch_mutex_cpu_relax(), which can be defined by each architecture that
    selects HAVE_ARCH_MUTEX_CPU_RELAX. The default is still cpu_relax(), so
    this patch should not affect other architectures than System z for now.

    Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
    ---
    arch/Kconfig | 3 +++
    arch/s390/Kconfig | 1 +
    arch/s390/include/asm/mutex.h | 2 ++
    include/linux/mutex.h | 4 ++++
    kernel/mutex.c | 2 +-
    kernel/sched.c | 2 +-
    6 files changed, 12 insertions(+), 2 deletions(-)

    --- a/arch/Kconfig
    +++ b/arch/Kconfig
    @@ -158,4 +158,7 @@ config HAVE_PERF_EVENTS_NMI
    subsystem. Also has support for calculating CPU cycle events
    to determine how many clock cycles in a given period.

    +config HAVE_ARCH_MUTEX_CPU_RELAX
    + bool
    +
    source "kernel/gcov/Kconfig"
    --- a/arch/s390/Kconfig
    +++ b/arch/s390/Kconfig
    @@ -100,6 +100,7 @@ config S390
    select HAVE_KERNEL_BZIP2
    select HAVE_KERNEL_LZMA
    select HAVE_KERNEL_LZO
    + select HAVE_ARCH_MUTEX_CPU_RELAX
    select ARCH_INLINE_SPIN_TRYLOCK
    select ARCH_INLINE_SPIN_TRYLOCK_BH
    select ARCH_INLINE_SPIN_LOCK
    --- a/arch/s390/include/asm/mutex.h
    +++ b/arch/s390/include/asm/mutex.h
    @@ -7,3 +7,5 @@
    */

    #include <asm-generic/mutex-dec.h>
    +
    +#define arch_mutex_cpu_relax() barrier()
    --- a/include/linux/mutex.h
    +++ b/include/linux/mutex.h
    @@ -160,4 +160,8 @@ extern int mutex_trylock(struct mutex *l
    extern void mutex_unlock(struct mutex *lock);
    extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);

    +#ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX
    +#define arch_mutex_cpu_relax() cpu_relax()
    +#endif
    +
    #endif
    --- a/kernel/mutex.c
    +++ b/kernel/mutex.c
    @@ -199,7 +199,7 @@ __mutex_lock_common(struct mutex *lock,
    * memory barriers as we'll eventually observe the right
    * values at the cost of a few extra spins.
    */
    - cpu_relax();
    + arch_mutex_cpu_relax();
    }
    #endif
    spin_lock_mutex(&lock->wait_lock, flags);
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -3888,7 +3888,7 @@ int mutex_spin_on_owner(struct mutex *lo
    if (task_thread_info(rq->curr) != owner || need_resched())
    return 0;

    - cpu_relax();
    + arch_mutex_cpu_relax();
    }

    return 1;



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