Commit a3127336 authored by Huang Ying's avatar Huang Ying Committed by Ingo Molnar
Browse files

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: default avatarHuang Ying <ying.huang@intel.com>
Acked-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@elte.hu>
parent 2c30245c

include/linux/llist.h

+5 −2
Original line number Diff line number Diff line
@@ -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);

	}

}



/**

lib/llist.c

+10 −4
Original line number Diff line number Diff line
@@ -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;

}

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