Re: [PATCH v7 3/6] MCS Lock: Move mcs_lock/unlock function into itsown file

[Paul E. McKenney]

On Thu, Jan 16, 2014 at 04:08:24PM -0800, Tim Chen wrote:
> The following changes are made:
> 
> 1) Create a new mcs_spinlock.c file to contain the
>    mcs_spin_lock() and mcs_spin_unlock() function.
> 2) Include a number of prerequisite header files and define
>    arch_mutex_cpu_relax(), if not previously defined so the
>    mcs functions can be compiled for multiple architecture without
>    causing problems.
> 
> From: Waiman Long <Waiman.Long@xxxxxx>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

> ---
>  include/linux/mcs_spinlock.h                       | 56 ++--------------------
>  kernel/locking/Makefile                            |  6 +--
>  .../locking/mcs_spinlock.c                         | 33 ++++++-------
>  3 files changed, 24 insertions(+), 71 deletions(-)
>  copy include/linux/mcs_spinlock.h => kernel/locking/mcs_spinlock.c (75%)
> 
> diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> index 96f14299..d54bb23 100644
> --- a/include/linux/mcs_spinlock.h
> +++ b/include/linux/mcs_spinlock.h
> @@ -17,57 +17,9 @@ struct mcs_spinlock {
>  	int locked; /* 1 if lock acquired */
>  };
> 
> -/*
> - * In order to acquire the lock, the caller should declare a local node and
> - * pass a reference of the node to this function in addition to the lock.
> - * If the lock has already been acquired, then this will proceed to spin
> - * on this node->locked until the previous lock holder sets the node->locked
> - * in mcs_spin_unlock().
> - *
> - * We don't inline mcs_spin_lock() so that perf can correctly account for the
> - * time spent in this lock function.
> - */
> -static noinline
> -void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> -{
> -	struct mcs_spinlock *prev;
> -
> -	/* Init node */
> -	node->locked = 0;
> -	node->next   = NULL;
> -
> -	prev = xchg(lock, node);
> -	if (likely(prev == NULL)) {
> -		/* Lock acquired */
> -		return;
> -	}
> -	ACCESS_ONCE(prev->next) = node;
> -	smp_wmb();
> -	/* Wait until the lock holder passes the lock down */
> -	while (!ACCESS_ONCE(node->locked))
> -		arch_mutex_cpu_relax();
> -}
> -
> -/*
> - * Releases the lock. The caller should pass in the corresponding node that
> - * was used to acquire the lock.
> - */
> -static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> -{
> -	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
> -
> -	if (likely(!next)) {
> -		/*
> -		 * Release the lock by setting it to NULL
> -		 */
> -		if (likely(cmpxchg(lock, node, NULL) == node))
> -			return;
> -		/* Wait until the next pointer is set */
> -		while (!(next = ACCESS_ONCE(node->next)))
> -			arch_mutex_cpu_relax();
> -	}
> -	ACCESS_ONCE(next->locked) = 1;
> -	smp_wmb();
> -}
> +extern
> +void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node);
> +extern
> +void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node);
> 
>  #endif /* __LINUX_MCS_SPINLOCK_H */
> diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
> index baab8e5..20d9d5c 100644
> --- a/kernel/locking/Makefile
> +++ b/kernel/locking/Makefile
> @@ -13,12 +13,12 @@ obj-$(CONFIG_LOCKDEP) += lockdep.o
>  ifeq ($(CONFIG_PROC_FS),y)
>  obj-$(CONFIG_LOCKDEP) += lockdep_proc.o
>  endif
> -obj-$(CONFIG_SMP) += spinlock.o
> -obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
> +obj-$(CONFIG_SMP) += spinlock.o mcs_spinlock.o
> +obj-$(CONFIG_PROVE_LOCKING) += spinlock.o mcs_spinlock.o
>  obj-$(CONFIG_RT_MUTEXES) += rtmutex.o
>  obj-$(CONFIG_DEBUG_RT_MUTEXES) += rtmutex-debug.o
>  obj-$(CONFIG_RT_MUTEX_TESTER) += rtmutex-tester.o
> -obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
> +obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o mcs_spinlock.o
>  obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock_debug.o
>  obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
>  obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
> diff --git a/include/linux/mcs_spinlock.h b/kernel/locking/mcs_spinlock.c
> similarity index 75%
> copy from include/linux/mcs_spinlock.h
> copy to kernel/locking/mcs_spinlock.c
> index 96f14299..44fb092 100644
> --- a/include/linux/mcs_spinlock.h
> +++ b/kernel/locking/mcs_spinlock.c
> @@ -1,7 +1,5 @@
>  /*
> - * MCS lock defines
> - *
> - * This file contains the main data structure and API definitions of MCS lock.
> + * MCS lock
>   *
>   * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
>   * with the desirable properties of being fair, and with each cpu trying
> @@ -9,13 +7,20 @@
>   * It avoids expensive cache bouncings that common test-and-set spin-lock
>   * implementations incur.
>   */
> -#ifndef __LINUX_MCS_SPINLOCK_H
> -#define __LINUX_MCS_SPINLOCK_H
> +/*
> + * asm/processor.h may define arch_mutex_cpu_relax().
> + * If it is not defined, cpu_relax() will be used.
> + */
> +#include <asm/barrier.h>
> +#include <asm/cmpxchg.h>
> +#include <asm/processor.h>
> +#include <linux/compiler.h>
> +#include <linux/mcs_spinlock.h>
> +#include <linux/export.h>
> 
> -struct mcs_spinlock {
> -	struct mcs_spinlock *next;
> -	int locked; /* 1 if lock acquired */
> -};
> +#ifndef arch_mutex_cpu_relax
> +# define arch_mutex_cpu_relax() cpu_relax()
> +#endif
> 
>  /*
>   * In order to acquire the lock, the caller should declare a local node and
> @@ -23,11 +28,7 @@ struct mcs_spinlock {
>   * If the lock has already been acquired, then this will proceed to spin
>   * on this node->locked until the previous lock holder sets the node->locked
>   * in mcs_spin_unlock().
> - *
> - * We don't inline mcs_spin_lock() so that perf can correctly account for the
> - * time spent in this lock function.
>   */
> -static noinline
>  void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  {
>  	struct mcs_spinlock *prev;
> @@ -47,12 +48,13 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  	while (!ACCESS_ONCE(node->locked))
>  		arch_mutex_cpu_relax();
>  }
> +EXPORT_SYMBOL_GPL(mcs_spin_lock);
> 
>  /*
>   * Releases the lock. The caller should pass in the corresponding node that
>   * was used to acquire the lock.
>   */
> -static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> +void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  {
>  	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
> 
> @@ -69,5 +71,4 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
>  	ACCESS_ONCE(next->locked) = 1;
>  	smp_wmb();
>  }
> -
> -#endif /* __LINUX_MCS_SPINLOCK_H */
> +EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> -- 
> 1.7.11.7
> 
> 
> 

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