idr: make idr_remove rcu-safe

Introduce the free_layer() routine: it is the one that actually frees memory
after a grace period has elapsed.
Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
Reviewed-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Jim Houston <jim.houston@comcast.net>
Cc: Pierre Peiffer <peifferp@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/idr.c

@@ -52,6 +52,19 @@ static struct idr_layer *get_from_free_list(struct idr *idp)
 	return(p);
 }
 
+static void idr_layer_rcu_free(struct rcu_head *head)
+{
+	struct idr_layer *layer;
+
+	layer = container_of(head, struct idr_layer, rcu_head);
+	kmem_cache_free(idr_layer_cache, layer);
+}
+
+static inline void free_layer(struct idr_layer *p)
+{
+	call_rcu(&p->rcu_head, idr_layer_rcu_free);
+}
+
 /* only called when idp->lock is held */
 static void __move_to_free_list(struct idr *idp, struct idr_layer *p)
 {
@@ -331,6 +344,7 @@ static void sub_remove(struct idr *idp, int shift, int id)
 	struct idr_layer *p = idp->top;
 	struct idr_layer **pa[MAX_LEVEL];
 	struct idr_layer ***paa = &pa[0];
+	struct idr_layer *to_free;
 	int n;
 
 	*paa = NULL;
@@ -346,13 +360,18 @@ static void sub_remove(struct idr *idp, int shift, int id)
 	n = id & IDR_MASK;
 	if (likely(p != NULL && test_bit(n, &p->bitmap))){
 		__clear_bit(n, &p->bitmap);
-		p->ary[n] = NULL;
+		rcu_assign_pointer(p->ary[n], NULL);
+		to_free = NULL;
 		while(*paa && ! --((**paa)->count)){
-			move_to_free_list(idp, **paa);
+			if (to_free)
+				free_layer(to_free);
+			to_free = **paa;
 			**paa-- = NULL;
 		}
 		if (!*paa)
 			idp->layers = 0;
+		if (to_free)
+			free_layer(to_free);
 	} else
 		idr_remove_warning(id);
 }
@@ -365,22 +384,34 @@ static void sub_remove(struct idr *idp, int shift, int id)
 void idr_remove(struct idr *idp, int id)
 {
 	struct idr_layer *p;
+	struct idr_layer *to_free;
 
 	/* Mask off upper bits we don't use for the search. */
 	id &= MAX_ID_MASK;
 
 	sub_remove(idp, (idp->layers - 1) * IDR_BITS, id);
 	if (idp->top && idp->top->count == 1 && (idp->layers > 1) &&
-	    idp->top->ary[0]) {  // We can drop a layer
-
+	    idp->top->ary[0]) {
+		/*
+		 * Single child at leftmost slot: we can shrink the tree.
+		 * This level is not needed anymore since when layers are
+		 * inserted, they are inserted at the top of the existing
+		 * tree.
+		 */
+		to_free = idp->top;
 		p = idp->top->ary[0];
-		idp->top->bitmap = idp->top->count = 0;
-		move_to_free_list(idp, idp->top);
-		idp->top = p;
+		rcu_assign_pointer(idp->top, p);
 		--idp->layers;
+		to_free->bitmap = to_free->count = 0;
+		free_layer(to_free);
 	}
 	while (idp->id_free_cnt >= IDR_FREE_MAX) {
 		p = get_from_free_list(idp);
+		/*
+		 * Note: we don't call the rcu callback here, since the only
+		 * layers that fall into the freelist are those that have been
+		 * preallocated.
+		 */
 		kmem_cache_free(idr_layer_cache, p);
 	}
 	return;
@@ -421,15 +452,13 @@ void idr_remove_all(struct idr *idp)
 
 		id += 1 << n;
 		while (n < fls(id)) {
-			if (p) {
-				memset(p, 0, sizeof *p);
-				move_to_free_list(idp, p);
-			}
+			if (p)
+				free_layer(p);
 			n += IDR_BITS;
 			p = *--paa;
 		}
 	}
-	idp->top = NULL;
+	rcu_assign_pointer(idp->top, NULL);
 	idp->layers = 0;
 }
 EXPORT_SYMBOL(idr_remove_all);
@@ -546,7 +575,7 @@ EXPORT_SYMBOL(idr_for_each);
  * A -ENOENT return indicates that @id was not found.
  * A -EINVAL return indicates that @id was not within valid constraints.
  *
- * The caller must serialize vs idr_find(), idr_get_new(), and idr_remove().
+ * The caller must serialize with writers.
  */
 void *idr_replace(struct idr *idp, void *ptr, int id)
 {
@@ -572,7 +601,7 @@ void *idr_replace(struct idr *idp, void *ptr, int id)
 		return ERR_PTR(-ENOENT);
 
 	old_p = p->ary[n];
-	p->ary[n] = ptr;
+	rcu_assign_pointer(p->ary[n], ptr);
 
 	return old_p;
 }