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


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