linux-kernel - [PATCH 1/3] idr: make idr_get_next() good for rcu_read

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Message-ID: <alpine.LSU.2.00.1201191247210.29542@eggly.anvils>
Date:	Thu, 19 Jan 2012 12:48:48 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Tejun Heo <tj@...nel.org>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] 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@...gle.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Acked-by: Li Zefan <lizf@...fujitsu.com>
---
 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, *@...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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