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llist: Move cpu_relax() to after the cmpxchg()

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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>
This commit is contained in:
Huang Ying 2011-09-08 14:00:44 +08:00 committed by Ingo Molnar
parent 2c30245c65
commit a3127336b7
2 changed files with 15 additions and 6 deletions
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7
include/linux/llist.h
View File

@ -148,11 +148,14 @@ static inline void llist_add(struct llist_node *new, struct llist_head *head)
struct llist_node *entry, *old_entry;
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);
}
}
/**

14
lib/llist.c
View File

@ -41,11 +41,14 @@ void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
struct llist_node *entry, *old_entry;
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);
}
}
EXPORT_SYMBOL_GPL(llist_add_batch);
@ -68,13 +71,16 @@ struct llist_node *llist_del_first(struct llist_head *head)
struct llist_node *entry, *old_entry, *next;
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;
}