[PATCH 1/3] idr: make idr_get_next() good for rcu_read_lock()

[Hugh Dickins]

Make one small adjustment to idr_get_next(): take the height from the
top layer (stable under RCU) instead of from the root (unprotected by
RCU), as idr_find() does: so that it can be used with RCU locking.
Copied comment on RCU locking from idr_find().

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Acked-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
---
 lib/idr.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- 3.2.0+.orig/lib/idr.c	2012-01-04 15:55:44.000000000 -0800
+++ 3.2.0+/lib/idr.c	2012-01-19 11:55:28.780206713 -0800
@@ -595,8 +595,10 @@ EXPORT_SYMBOL(idr_for_each);
  * Returns pointer to registered object with id, which is next number to
  * given id. After being looked up, *@nextidp will be updated for the next
  * iteration.
+ *
+ * This function can be called under rcu_read_lock(), given that the leaf
+ * pointers lifetimes are correctly managed.
  */
-
 void *idr_get_next(struct idr *idp, int *nextidp)
 {
 	struct idr_layer *p, *pa[MAX_LEVEL];
@@ -605,11 +607,11 @@ void *idr_get_next(struct idr *idp, int
 	int n, max;
 
 	/* find first ent */
-	n = idp->layers * IDR_BITS;
-	max = 1 << n;
 	p = rcu_dereference_raw(idp->top);
 	if (!p)
 		return NULL;
+	n = (p->layer + 1) * IDR_BITS;
+	max = 1 << n;
 
 	while (id < max) {
 		while (n > 0 && p) {
--
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