lib/idr.c: fix rcu related race with idr_find

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@colorfullife.com>
Cc: Clement Calmels <cboulte@gmail.com>
Cc: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Pierre Peiffer <peifferp@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 14 insertions, 3 deletions

diff --git a/include/linux/idr.h b/include/linux/idr.h
index fa035f96f2a3..dd846df8cd32 100644
--- 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;
 };
diff --git a/lib/idr.c b/lib/idr.c
index e728c7fccc4d..7a785a0c2ea0 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -185,6 +185,7 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa)
                         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 *ptr, int id)
         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;