Messages in this thread Patch in this message | | | Date | Sat, 15 Oct 2005 10:26:49 +1000 | Subject | [LIST] Add missing rcu_dereference on first element | From | Herbert Xu <> |
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Hi:
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>
Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt diff --git a/include/linux/list.h b/include/linux/list.h --- a/include/linux/list.h +++ b/include/linux/list.h @@ -442,12 +442,15 @@ static inline void list_splice_init(stru * 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; \ + pos = rcu_dereference(pos), \ + prefetch(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 +464,9 @@ static inline void list_splice_init(stru * 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 +478,11 @@ static inline void list_splice_init(stru * 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); \ + pos = rcu_dereference(pos), \ + prefetch(pos->member.next), &pos->member != (head); \ + pos = list_entry(pos->member.next, typeof(*pos), member)) /** @@ -492,8 +496,10 @@ static inline void list_splice_init(stru * 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; \ + (pos) = rcu_dereference((pos)), \ + prefetch((pos)->next), (pos) != (head); \ + (pos) = (pos)->next) /* * Double linked lists with a single pointer list head. @@ -696,8 +702,9 @@ static inline void hlist_add_after_rcu(s 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 +769,9 @@ static inline void hlist_add_after_rcu(s */ #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" | |