On top of rcu-add-a-prefetch-in-rcu_do_batch.patch
rcu_do_batch:
struct rcu_head *next, *list;
while (list) {
next = list->next; <------ [1]
list->func(list);
list = next;
}
We can't trust *list after list->func() call, that is why we load list->next
beforehand. However I suspect in theory this is not enough, suppose that
- [1] is stalled
- list->func() marks *list as unused in some way
- another CPU re-uses this rcu_head and dirties it
- [1] completes and gets a wrong result
This means we need a barrier in between. mb() looks more suitable, but I think
rmb() should suffice.
Signed-off-by: Oleg Nesterov <[email protected]>
--- 19-rc6/kernel/rcupdate.c~rdp 2006-12-02 20:46:03.000000000 +0300
+++ 19-rc6/kernel/rcupdate.c 2006-12-02 21:04:12.000000000 +0300
@@ -236,6 +236,8 @@ static void rcu_do_batch(struct rcu_data
list = rdp->donelist;
while (list) {
next = list->next;
+ /* complete the load above before we call ->func() */
+ smp_rmb();
prefetch(next);
list->func(list);
list = next;
-
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