linux-kernel - [ 113/153] idr: make idr_get_next() good for rcu_read

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Message-Id: <20130304033718.706151947@decadent.org.uk>
Date:	Mon, 04 Mar 2013 03:39:00 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [ 113/153] idr: make idr_get_next() good for rcu_read_lock()

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Hugh Dickins <hughd@...gle.com>

commit 9f7de8275b46d9d11b1505adbfe6c2bb48df4741 upstream.

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@...gle.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Acked-by: Li Zefan <lizf@...fujitsu.com>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Acked-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 lib/idr.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- 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, *@...tidp 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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