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-rw-r--r--lib/radix-tree.c83
1 files changed, 58 insertions, 25 deletions
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 6f412ab4c24f..5086bb962b4d 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -82,6 +82,16 @@ struct radix_tree_preload {
};
static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };
+static inline void *ptr_to_indirect(void *ptr)
+{
+ return (void *)((unsigned long)ptr | RADIX_TREE_INDIRECT_PTR);
+}
+
+static inline void *indirect_to_ptr(void *ptr)
+{
+ return (void *)((unsigned long)ptr & ~RADIX_TREE_INDIRECT_PTR);
+}
+
static inline gfp_t root_gfp_mask(struct radix_tree_root *root)
{
return root->gfp_mask & __GFP_BITS_MASK;
@@ -265,7 +275,7 @@ static int radix_tree_extend(struct radix_tree_root *root, unsigned long index)
return -ENOMEM;
/* Increase the height. */
- node->slots[0] = radix_tree_indirect_to_ptr(root->rnode);
+ node->slots[0] = indirect_to_ptr(root->rnode);
/* Propagate the aggregated tag info into the new root */
for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
@@ -276,7 +286,7 @@ static int radix_tree_extend(struct radix_tree_root *root, unsigned long index)
newheight = root->height+1;
node->height = newheight;
node->count = 1;
- node = radix_tree_ptr_to_indirect(node);
+ node = ptr_to_indirect(node);
rcu_assign_pointer(root->rnode, node);
root->height = newheight;
} while (height > root->height);
@@ -309,7 +319,7 @@ int radix_tree_insert(struct radix_tree_root *root,
return error;
}
- slot = radix_tree_indirect_to_ptr(root->rnode);
+ slot = indirect_to_ptr(root->rnode);
height = root->height;
shift = (height-1) * RADIX_TREE_MAP_SHIFT;
@@ -325,8 +335,7 @@ int radix_tree_insert(struct radix_tree_root *root,
rcu_assign_pointer(node->slots[offset], slot);
node->count++;
} else
- rcu_assign_pointer(root->rnode,
- radix_tree_ptr_to_indirect(slot));
+ rcu_assign_pointer(root->rnode, ptr_to_indirect(slot));
}
/* Go a level down */
@@ -374,7 +383,7 @@ static void *radix_tree_lookup_element(struct radix_tree_root *root,
return NULL;
return is_slot ? (void *)&root->rnode : node;
}
- node = radix_tree_indirect_to_ptr(node);
+ node = indirect_to_ptr(node);
height = node->height;
if (index > radix_tree_maxindex(height))
@@ -393,7 +402,7 @@ static void *radix_tree_lookup_element(struct radix_tree_root *root,
height--;
} while (height > 0);
- return is_slot ? (void *)slot:node;
+ return is_slot ? (void *)slot : indirect_to_ptr(node);
}
/**
@@ -455,7 +464,7 @@ void *radix_tree_tag_set(struct radix_tree_root *root,
height = root->height;
BUG_ON(index > radix_tree_maxindex(height));
- slot = radix_tree_indirect_to_ptr(root->rnode);
+ slot = indirect_to_ptr(root->rnode);
shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
while (height > 0) {
@@ -509,7 +518,7 @@ void *radix_tree_tag_clear(struct radix_tree_root *root,
shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
pathp->node = NULL;
- slot = radix_tree_indirect_to_ptr(root->rnode);
+ slot = indirect_to_ptr(root->rnode);
while (height > 0) {
int offset;
@@ -579,7 +588,7 @@ int radix_tree_tag_get(struct radix_tree_root *root,
if (!radix_tree_is_indirect_ptr(node))
return (index == 0);
- node = radix_tree_indirect_to_ptr(node);
+ node = indirect_to_ptr(node);
height = node->height;
if (index > radix_tree_maxindex(height))
@@ -666,7 +675,7 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
}
shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
- slot = radix_tree_indirect_to_ptr(root->rnode);
+ slot = indirect_to_ptr(root->rnode);
/*
* we fill the path from (root->height - 2) to 0, leaving the index at
@@ -897,7 +906,7 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
results[0] = node;
return 1;
}
- node = radix_tree_indirect_to_ptr(node);
+ node = indirect_to_ptr(node);
max_index = radix_tree_maxindex(node->height);
@@ -916,7 +925,8 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
slot = *(((void ***)results)[ret + i]);
if (!slot)
continue;
- results[ret + nr_found] = rcu_dereference_raw(slot);
+ results[ret + nr_found] =
+ indirect_to_ptr(rcu_dereference_raw(slot));
nr_found++;
}
ret += nr_found;
@@ -965,7 +975,7 @@ radix_tree_gang_lookup_slot(struct radix_tree_root *root, void ***results,
results[0] = (void **)&root->rnode;
return 1;
}
- node = radix_tree_indirect_to_ptr(node);
+ node = indirect_to_ptr(node);
max_index = radix_tree_maxindex(node->height);
@@ -1090,7 +1100,7 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
results[0] = node;
return 1;
}
- node = radix_tree_indirect_to_ptr(node);
+ node = indirect_to_ptr(node);
max_index = radix_tree_maxindex(node->height);
@@ -1109,7 +1119,8 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
slot = *(((void ***)results)[ret + i]);
if (!slot)
continue;
- results[ret + nr_found] = rcu_dereference_raw(slot);
+ results[ret + nr_found] =
+ indirect_to_ptr(rcu_dereference_raw(slot));
nr_found++;
}
ret += nr_found;
@@ -1159,7 +1170,7 @@ radix_tree_gang_lookup_tag_slot(struct radix_tree_root *root, void ***results,
results[0] = (void **)&root->rnode;
return 1;
}
- node = radix_tree_indirect_to_ptr(node);
+ node = indirect_to_ptr(node);
max_index = radix_tree_maxindex(node->height);
@@ -1195,7 +1206,7 @@ static inline void radix_tree_shrink(struct radix_tree_root *root)
void *newptr;
BUG_ON(!radix_tree_is_indirect_ptr(to_free));
- to_free = radix_tree_indirect_to_ptr(to_free);
+ to_free = indirect_to_ptr(to_free);
/*
* The candidate node has more than one child, or its child
@@ -1208,16 +1219,39 @@ static inline void radix_tree_shrink(struct radix_tree_root *root)
/*
* We don't need rcu_assign_pointer(), since we are simply
- * moving the node from one part of the tree to another. If
- * it was safe to dereference the old pointer to it
+ * moving the node from one part of the tree to another: if it
+ * was safe to dereference the old pointer to it
* (to_free->slots[0]), it will be safe to dereference the new
- * one (root->rnode).
+ * one (root->rnode) as far as dependent read barriers go.
*/
newptr = to_free->slots[0];
if (root->height > 1)
- newptr = radix_tree_ptr_to_indirect(newptr);
+ newptr = ptr_to_indirect(newptr);
root->rnode = newptr;
root->height--;
+
+ /*
+ * We have a dilemma here. The node's slot[0] must not be
+ * NULLed in case there are concurrent lookups expecting to
+ * find the item. However if this was a bottom-level node,
+ * then it may be subject to the slot pointer being visible
+ * to callers dereferencing it. If item corresponding to
+ * slot[0] is subsequently deleted, these callers would expect
+ * their slot to become empty sooner or later.
+ *
+ * For example, lockless pagecache will look up a slot, deref
+ * the page pointer, and if the page is 0 refcount it means it
+ * was concurrently deleted from pagecache so try the deref
+ * again. Fortunately there is already a requirement for logic
+ * to retry the entire slot lookup -- the indirect pointer
+ * problem (replacing direct root node with an indirect pointer
+ * also results in a stale slot). So tag the slot as indirect
+ * to force callers to retry.
+ */
+ if (root->height == 0)
+ *((unsigned long *)&to_free->slots[0]) |=
+ RADIX_TREE_INDIRECT_PTR;
+
radix_tree_node_free(to_free);
}
}
@@ -1254,7 +1288,7 @@ void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
root->rnode = NULL;
goto out;
}
- slot = radix_tree_indirect_to_ptr(slot);
+ slot = indirect_to_ptr(slot);
shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
pathp->node = NULL;
@@ -1296,8 +1330,7 @@ void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
radix_tree_node_free(to_free);
if (pathp->node->count) {
- if (pathp->node ==
- radix_tree_indirect_to_ptr(root->rnode))
+ if (pathp->node == indirect_to_ptr(root->rnode))
radix_tree_shrink(root);
goto out;
}
Copyright © 1996 – 2023 Jason A. Donenfeld. All Rights Reverse Engineered.