llist: Move cpu_relax() to after the cmpxchg()
If in llist_add()/etc. functions the first cmpxchg() call succeeds, it is not necessary to use cpu_relax() before the cmpxchg(). So cpu_relax() in a busy loop involving cmpxchg() should go after cmpxchg() instead of before that. This patch fixes this for all involved llist functions. Signed-off-by: Huang Ying <ying.huang@intel.com> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/r/1315461646-1379-4-git-send-email-ying.huang@intel.com Signed-off-by: Ingo Molnar <mingo@elte.hu>
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@ -148,11 +148,14 @@ static inline void llist_add(struct llist_node *new, struct llist_head *head)
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struct llist_node *entry, *old_entry;
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struct llist_node *entry, *old_entry;
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entry = head->first;
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entry = head->first;
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do {
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for (;;) {
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old_entry = entry;
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old_entry = entry;
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new->next = entry;
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new->next = entry;
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entry = cmpxchg(&head->first, old_entry, new);
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if (entry == old_entry)
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break;
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cpu_relax();
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cpu_relax();
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} while ((entry = cmpxchg(&head->first, old_entry, new)) != old_entry);
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}
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}
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}
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/**
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/**
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14
lib/llist.c
14
lib/llist.c
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@ -41,11 +41,14 @@ void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
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struct llist_node *entry, *old_entry;
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struct llist_node *entry, *old_entry;
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entry = head->first;
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entry = head->first;
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do {
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for (;;) {
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old_entry = entry;
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old_entry = entry;
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new_last->next = entry;
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new_last->next = entry;
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entry = cmpxchg(&head->first, old_entry, new_first);
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if (entry == old_entry)
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break;
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cpu_relax();
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cpu_relax();
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} while ((entry = cmpxchg(&head->first, old_entry, new_first)) != old_entry);
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}
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}
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}
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EXPORT_SYMBOL_GPL(llist_add_batch);
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EXPORT_SYMBOL_GPL(llist_add_batch);
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@ -68,13 +71,16 @@ struct llist_node *llist_del_first(struct llist_head *head)
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struct llist_node *entry, *old_entry, *next;
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struct llist_node *entry, *old_entry, *next;
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entry = head->first;
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entry = head->first;
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do {
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for (;;) {
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if (entry == NULL)
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if (entry == NULL)
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return NULL;
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return NULL;
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old_entry = entry;
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old_entry = entry;
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next = entry->next;
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next = entry->next;
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entry = cmpxchg(&head->first, old_entry, next);
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if (entry == old_entry)
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break;
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cpu_relax();
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cpu_relax();
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} while ((entry = cmpxchg(&head->first, old_entry, next)) != old_entry);
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}
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return entry;
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return entry;
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}
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}
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