3 files changed, 91 insertions, 57 deletions

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index a39cbed9ee17..ab2baa5c4884 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -34,19 +34,13 @@
  * needed for RCU lookups (because root->height is unreliable). The only
  * time callers need worry about this is when doing a lookup_slot under
  * RCU.
+ *
+ * Indirect pointer in fact is also used to tag the last pointer of a node
+ * when it is shrunk, before we rcu free the node. See shrink code for
+ * details.
  */
 #define RADIX_TREE_INDIRECT_PTR 1
-#define RADIX_TREE_RETRY ((void *)-1UL)
-
-static inline void *radix_tree_ptr_to_indirect(void *ptr)
-{
-        return (void *)((unsigned long)ptr | RADIX_TREE_INDIRECT_PTR);
-}
 
-static inline void *radix_tree_indirect_to_ptr(void *ptr)
-{
-        return (void *)((unsigned long)ptr & ~RADIX_TREE_INDIRECT_PTR);
-}
 #define radix_tree_indirect_to_ptr(ptr) \
         radix_tree_indirect_to_ptr((void __force *)(ptr))
 
@@ -140,16 +134,29 @@ do {									\
  *              removed.
  *
  * For use with radix_tree_lookup_slot().  Caller must hold tree at least read
- * locked across slot lookup and dereference.  More likely, will be used with
- * radix_tree_replace_slot(), as well, so caller will hold tree write locked.
+ * locked across slot lookup and dereference. Not required if write lock is
+ * held (ie. items cannot be concurrently inserted).
+ *
+ * radix_tree_deref_retry must be used to confirm validity of the pointer if
+ * only the read lock is held.
  */
 static inline void *radix_tree_deref_slot(void **pslot)
 {
-        void *ret = rcu_dereference(*pslot);
-        if (unlikely(radix_tree_is_indirect_ptr(ret)))
-                ret = RADIX_TREE_RETRY;
-        return ret;
+        return rcu_dereference(*pslot);
 }
+
+/**
+ * radix_tree_deref_retry       - check radix_tree_deref_slot
+ * @arg:        pointer returned by radix_tree_deref_slot
+ * Returns:     0 if retry is not required, otherwise retry is required
+ *
+ * radix_tree_deref_retry must be used with radix_tree_deref_slot.
+ */
+static inline int radix_tree_deref_retry(void *arg)
+{
+        return unlikely((unsigned long)arg & RADIX_TREE_INDIRECT_PTR);
+}
+
 /**
  * radix_tree_replace_slot      - replace item in a slot
  * @pslot:      pointer to slot, returned by radix_tree_lookup_slot
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;
                 }
diff --git a/mm/filemap.c b/mm/filemap.c
index 4ee2e998e937..ea89840fc65f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -644,7 +644,9 @@ repeat:
         pagep = radix_tree_lookup_slot(&mapping->page_tree, offset);
         if (pagep) {
                 page = radix_tree_deref_slot(pagep);
-                if (unlikely(!page || page == RADIX_TREE_RETRY))
+                if (unlikely(!page))
+                        goto out;
+                if (radix_tree_deref_retry(page))
                         goto repeat;
 
                 if (!page_cache_get_speculative(page))
@@ -660,6 +662,7 @@ repeat:
                         goto repeat;
                 }
         }
+out:
         rcu_read_unlock();
 
         return page;
@@ -777,12 +780,11 @@ repeat:
                 page = radix_tree_deref_slot((void **)pages[i]);
                 if (unlikely(!page))
                         continue;
-                /*
-                 * this can only trigger if nr_found == 1, making livelock
-                 * a non issue.
-                 */
-                if (unlikely(page == RADIX_TREE_RETRY))
+                if (radix_tree_deref_retry(page)) {
+                        if (ret)
+                                start = pages[ret-1]->index;
                         goto restart;
+                }
 
                 if (!page_cache_get_speculative(page))
                         goto repeat;
@@ -830,11 +832,7 @@ repeat:
                 page = radix_tree_deref_slot((void **)pages[i]);
                 if (unlikely(!page))
                         continue;
-                /*
-                 * this can only trigger if nr_found == 1, making livelock
-                 * a non issue.
-                 */
-                if (unlikely(page == RADIX_TREE_RETRY))
+                if (radix_tree_deref_retry(page))
                         goto restart;
 
                 if (page->mapping == NULL || page->index != index)
@@ -887,11 +885,7 @@ repeat:
                 page = radix_tree_deref_slot((void **)pages[i]);
                 if (unlikely(!page))
                         continue;
-                /*
-                 * this can only trigger if nr_found == 1, making livelock
-                 * a non issue.
-                 */
-                if (unlikely(page == RADIX_TREE_RETRY))
+                if (radix_tree_deref_retry(page))
                         goto restart;
 
                 if (!page_cache_get_speculative(page))