Re: [PATCH -mm 3/4] llist, Move cpu_relax after cmpxchg

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Huang Ying (ying.huang@intel.com) wrote:
> If the first cmpxchg calling succeeds, it is not necessary to use
> cpu_relax before cmpxchg.  So cpu_relax in a busy loop involving
> cmpxchg should go after cmpxchg instead of before that.  This patch
> fixes this in llist.
>

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Thanks!

MAthieu
 
> Signed-off-by: Huang Ying <ying.huang@intel.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
>  include/linux/llist.h |   21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/llist.h
> +++ b/include/linux/llist.h
> @@ -156,11 +156,14 @@ static inline void llist_add(struct llis
>  	CHECK_NMI_SAFE_CMPXCHG();
>  
>  	entry = head->first;
> -	do {
> +	for (;;) {
>  		old_entry = entry;
>  		new->next = entry;
> +		entry = cmpxchg(&head->first, old_entry, new);
> +		if (entry == old_entry)
> +			break;
>  		cpu_relax();
> -	} while ((entry = cmpxchg(&head->first, old_entry, new)) != old_entry);
> +	}
>  }
>  
>  /**
> @@ -178,11 +181,14 @@ static inline void llist_add_batch(struc
>  	CHECK_NMI_SAFE_CMPXCHG();
>  
>  	entry = head->first;
> -	do {
> +	for (;;) {
>  		old_entry = entry;
>  		new_last->next = entry;
> +		entry = cmpxchg(&head->first, old_entry, new_first);
> +		if (entry == old_entry)
> +			break;
>  		cpu_relax();
> -	} while ((entry = cmpxchg(&head->first, old_entry, new_first)) != old_entry);
> +	}
>  }
>  
>  /**
> @@ -206,13 +212,16 @@ static inline struct llist_node *llist_d
>  	CHECK_NMI_SAFE_CMPXCHG();
>  
>  	entry = head->first;
> -	do {
> +	for (;;) {
>  		if (entry == NULL)
>  			return NULL;
>  		old_entry = entry;
>  		next = entry->next;
> +		entry = cmpxchg(&head->first, old_entry, next);
> +		if (entry == old_entry)
> +			break;
>  		cpu_relax();
> -	} while ((entry = cmpxchg(&head->first, old_entry, next)) != old_entry);
> +	}
>  
>  	return entry;
>  }

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com