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[PATCH] list: add missing rcu_dereference on first element 

It seems that all the list_*_rcu primitives are missing a memory barrier
on the very first dereference.  For example,

#define list_for_each_rcu(pos, head) \
	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
		pos = rcu_dereference(pos->next))

It will go something like:

	pos = (head)->next

	prefetch(pos->next)

	pos != (head)

	do stuff

We're missing a barrier here.

	pos = rcu_dereference(pos->next)

		fetch pos->next

		barrier given by rcu_dereference(pos->next)

		store pos

Without the missing barrier, the pos->next value may turn out to be stale.
In fact, if "do stuff" were also dereferencing pos and relying on
list_for_each_rcu to provide the barrier then it may also break.

So here is a patch to make sure that we have a barrier for the first
element in the list.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Herbert Xu 2005-10-16 20:29:20 -07:00 committed by Linus Torvalds
parent 3d80636a0d
commit b24d18aa74
1 changed files with 22 additions and 17 deletions
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39
include/linux/list.h
View File

@ -442,12 +442,14 @@ static inline void list_splice_init(struct list_head *list,
* as long as the traversal is guarded by rcu_read_lock().
*/
#define list_for_each_rcu(pos, head) \
for (pos = (head)->next; prefetch(pos->next), pos != (head); \
pos = rcu_dereference(pos->next))
for (pos = (head)->next; \
prefetch(rcu_dereference(pos)->next), pos != (head); \
pos = pos->next)
#define __list_for_each_rcu(pos, head) \
for (pos = (head)->next; pos != (head); \
pos = rcu_dereference(pos->next))
for (pos = (head)->next; \
rcu_dereference(pos) != (head); \
pos = pos->next)
/**
* list_for_each_safe_rcu - iterate over an rcu-protected list safe
@ -461,8 +463,9 @@ static inline void list_splice_init(struct list_head *list,
* as long as the traversal is guarded by rcu_read_lock().
*/
#define list_for_each_safe_rcu(pos, n, head) \
for (pos = (head)->next, n = pos->next; pos != (head); \
pos = rcu_dereference(n), n = pos->next)
for (pos = (head)->next; \
n = rcu_dereference(pos)->next, pos != (head); \
pos = n)
/**
* list_for_each_entry_rcu - iterate over rcu list of given type
@ -474,11 +477,11 @@ static inline void list_splice_init(struct list_head *list,
* the _rcu list-mutation primitives such as list_add_rcu()
* as long as the traversal is guarded by rcu_read_lock().
*/
#define list_for_each_entry_rcu(pos, head, member) \
for (pos = list_entry((head)->next, typeof(*pos), member); \
prefetch(pos->member.next), &pos->member != (head); \
pos = rcu_dereference(list_entry(pos->member.next, \
typeof(*pos), member)))
#define list_for_each_entry_rcu(pos, head, member) \
for (pos = list_entry((head)->next, typeof(*pos), member); \
prefetch(rcu_dereference(pos)->member.next), \
&pos->member != (head); \
pos = list_entry(pos->member.next, typeof(*pos), member))
/**
@ -492,8 +495,9 @@ static inline void list_splice_init(struct list_head *list,
* as long as the traversal is guarded by rcu_read_lock().
*/
#define list_for_each_continue_rcu(pos, head) \
for ((pos) = (pos)->next; prefetch((pos)->next), (pos) != (head); \
(pos) = rcu_dereference((pos)->next))
for ((pos) = (pos)->next; \
prefetch(rcu_dereference((pos))->next), (pos) != (head); \
(pos) = (pos)->next)
/*
* Double linked lists with a single pointer list head.
@ -696,8 +700,9 @@ static inline void hlist_add_after_rcu(struct hlist_node *prev,
pos = n)
#define hlist_for_each_rcu(pos, head) \
for ((pos) = (head)->first; pos && ({ prefetch((pos)->next); 1; }); \
(pos) = rcu_dereference((pos)->next))
for ((pos) = (head)->first; \
rcu_dereference((pos)) && ({ prefetch((pos)->next); 1; }); \
(pos) = (pos)->next)
/**
* hlist_for_each_entry - iterate over list of given type
@ -762,9 +767,9 @@ static inline void hlist_add_after_rcu(struct hlist_node *prev,
*/
#define hlist_for_each_entry_rcu(tpos, pos, head, member) \
for (pos = (head)->first; \
pos && ({ prefetch(pos->next); 1;}) && \
rcu_dereference(pos) && ({ prefetch(pos->next); 1;}) && \
({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
pos = rcu_dereference(pos->next))
pos = pos->next)
#else
#warning "don't include kernel headers in userspace"