[patch 021/104] lib/idr.c: fix rcu related race with idr_find

[Greg KH]

2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>

commit 6ff2d39b91aec3dcae951afa982059e3dd9b49dc upstream.

2nd part of the fixes needed for
http://bugzilla.kernel.org/show_bug.cgi?id=11796.

When the idr tree is either grown or shrunk, then the update to the number
of layers and the top pointer were not atomic.  This race caused crashes.

The attached patch fixes that by replicating the layers counter in each
layer, thus idr_find doesn't need idp->layers anymore.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Clement Calmels <cboulte@xxxxxxxxx>
Cc: Nadia Derbey <Nadia.Derbey@xxxxxxxx>
Cc: Pierre Peiffer <peifferp@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 include/linux/idr.h |    3 ++-
 lib/idr.c           |   14 ++++++++++++--
 2 files changed, 14 insertions(+), 3 deletions(-)

--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -52,13 +52,14 @@ struct idr_layer {
 	unsigned long		 bitmap; /* A zero bit means "space here" */
 	struct idr_layer	*ary[1<<IDR_BITS];
 	int			 count;	 /* When zero, we can release it */
+	int			 layer;	 /* distance from leaf */
 	struct rcu_head		 rcu_head;
 };
 
 struct idr {
 	struct idr_layer *top;
 	struct idr_layer *id_free;
-	int		  layers;
+	int		  layers; /* only valid without concurrent changes */
 	int		  id_free_cnt;
 	spinlock_t	  lock;
 };
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -185,6 +185,7 @@ static int sub_alloc(struct idr *idp, in
 			new = get_from_free_list(idp);
 			if (!new)
 				return -1;
+			new->layer = l-1;
 			rcu_assign_pointer(p->ary[m], new);
 			p->count++;
 		}
@@ -210,6 +211,7 @@ build_up:
 	if (unlikely(!p)) {
 		if (!(p = get_from_free_list(idp)))
 			return -1;
+		p->layer = 0;
 		layers = 1;
 	}
 	/*
@@ -237,6 +239,7 @@ build_up:
 		}
 		new->ary[0] = p;
 		new->count = 1;
+		new->layer = layers-1;
 		if (p->bitmap == IDR_FULL)
 			__set_bit(0, &new->bitmap);
 		p = new;
@@ -493,17 +496,21 @@ void *idr_find(struct idr *idp, int id)
 	int n;
 	struct idr_layer *p;
 
-	n = idp->layers * IDR_BITS;
 	p = rcu_dereference(idp->top);
+	if (!p)
+		return NULL;
+	n = (p->layer+1) * IDR_BITS;
 
 	/* Mask off upper bits we don't use for the search. */
 	id &= MAX_ID_MASK;
 
 	if (id >= (1 << n))
 		return NULL;
+	BUG_ON(n == 0);
 
 	while (n > 0 && p) {
 		n -= IDR_BITS;
+		BUG_ON(n != p->layer*IDR_BITS);
 		p = rcu_dereference(p->ary[(id >> n) & IDR_MASK]);
 	}
 	return((void *)p);
@@ -582,8 +589,11 @@ void *idr_replace(struct idr *idp, void 
 	int n;
 	struct idr_layer *p, *old_p;
 
-	n = idp->layers * IDR_BITS;
 	p = idp->top;
+	if (!p)
+		return ERR_PTR(-EINVAL);
+
+	n = (p->layer+1) * IDR_BITS;
 
 	id &= MAX_ID_MASK;
 

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