Re: [PATCH v7 5/6] MCS Lock: allow architectures to hook in tocontended paths

[Paul E. McKenney]

On Thu, Jan 16, 2014 at 04:08:31PM -0800, Tim Chen wrote:
> When contended, architectures may be able to reduce the polling overhead
> in ways which aren't expressible using a simple relax() primitive.
> 
> This patch allows architectures to hook into the mcs_{lock,unlock}
> functions for the contended cases only.
> 
> From: Will Deacon <will.deacon@xxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>

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

> ---
>  kernel/locking/mcs_spinlock.c | 47 +++++++++++++++++++++++++------------------
>  1 file changed, 27 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
> index 6cdc730..66d8883 100644
> --- a/kernel/locking/mcs_spinlock.c
> +++ b/kernel/locking/mcs_spinlock.c
> @@ -7,19 +7,34 @@
>   * It avoids expensive cache bouncings that common test-and-set spin-lock
>   * implementations incur.
>   */
> -/*
> - * 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/mutex.h>
>  #include <linux/export.h>
> 
> -#ifndef arch_mutex_cpu_relax
> -# define arch_mutex_cpu_relax() cpu_relax()
> +#ifndef arch_mcs_spin_lock_contended
> +/*
> + * Using smp_load_acquire() provides a memory barrier that ensures
> + * subsequent operations happen after the lock is acquired.
> + */
> +#define arch_mcs_spin_lock_contended(l)					\
> +	while (!(smp_load_acquire(l))) {				\
> +		arch_mutex_cpu_relax();					\
> +	}
> +#endif
> +
> +#ifndef arch_mcs_spin_unlock_contended
> +/*
> + * smp_store_release() provides a memory barrier to ensure all
> + * operations in the critical section has been completed before
> + * unlocking.
> + */
> +#define arch_mcs_spin_unlock_contended(l)				\
> +	smp_store_release((l), 1)
>  #endif
> 
>  /*
> @@ -43,13 +58,9 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  		return;
>  	}
>  	ACCESS_ONCE(prev->next) = node;
> -	/*
> -	 * Wait until the lock holder passes the lock down.
> -	 * Using smp_load_acquire() provides a memory barrier that
> -	 * ensures subsequent operations happen after the lock is acquired.
> -	 */
> -	while (!(smp_load_acquire(&node->locked)))
> -		arch_mutex_cpu_relax();
> +
> +	/* Wait until the lock holder passes the lock down. */
> +	arch_mcs_spin_lock_contended(&node->locked);
>  }
>  EXPORT_SYMBOL_GPL(mcs_spin_lock);
> 
> @@ -71,12 +82,8 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  		while (!(next = ACCESS_ONCE(node->next)))
>  			arch_mutex_cpu_relax();
>  	}
> -	/*
> -	 * Pass lock to next waiter.
> -	 * smp_store_release() provides a memory barrier to ensure
> -	 * all operations in the critical section has been completed
> -	 * before unlocking.
> -	 */
> -	smp_store_release(&next->locked, 1);
> +
> +	/* Pass lock to next waiter. */
> +	arch_mcs_spin_unlock_contended(&next->locked);
>  }
>  EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> -- 
> 1.7.11.7
> 
> 
> 

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