idr: make idr_get_next() good for rcu_read_lock()

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@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/idr.c b/lib/idr.c
index ed055b297c81c127dcbef79cb821f904936245c2..12499ba7967e08f5388835834c08c67dc7af5c24 100644 (file)
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -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 *nextidp)
        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) {