Re: [PATCH] Fixed a mismatch between the users of radix_tree and the implementation.
From: Salman Qazi
Date: Tue Aug 17 2010 - 17:28:38 EST
On Tue, Aug 17, 2010 at 8:23 AM, Nick Piggin <npiggin@xxxxxxxxx> wrote:
> On Mon, Aug 16, 2010 at 11:30:07AM -0700, Salman Qazi wrote:
>> Done.
>>
>> ---
>>
>> This matters for the lockless page cache, in particular find_get_pages implementation.
>>
>> In the following case, we get can get a deadlock:
>>
>> 0. The radix tree contains two items, one has the index 0.
>> 1. The reader (in this case find_get_pages) takes the rcu_read_lock.
>> 2. The reader acquires slot(s) for item(s) including the index 0 item.
>> 3. The non-zero index item is deleted, and as a consequence the other item
>> is moved to the root of the tree. The place where it used to be is
>> queued for deletion after the readers finish.
> 3b. The zero item is deleted, removing it from the direct slot,
> it remains in the rcu-delayed indirect node.
>> 4. The reader looks at the index 0 slot, and finds that the page has 0 ref count
>> 5. The reader looks at it again, hoping that the item will either be freed
>> or the ref count will increase. This never happens, as the slot it
>> is looking at will never be updated. Also, this slot can never be reclaimed
>> because the reader is holding rcu_read_lock and is in an infinite loop.
>
> Good catch. Increasing memory footprint really sucks actually. 5MB is a
> lot of memory, and it also means another pointer dereference on small
> files.
>
> I actually do go to a lot of trouble already to have direct pointers.
> Unfortunately this is another little complexity, but I really think it's
> worthwhile.
>
> How about something like this?
> --
> Index: linux-2.6/include/linux/radix-tree.h
> ===================================================================
> --- linux-2.6.orig/include/linux/radix-tree.h 2010-08-18 00:01:19.000000000 +1000
> +++ linux-2.6/include/linux/radix-tree.h 2010-08-18 00:56:32.000000000 +1000
> @@ -34,19 +34,12 @@
> * 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);
> -}
>
> static inline int radix_tree_is_indirect_ptr(void *ptr)
> {
> @@ -138,16 +131,29 @@ 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
> Index: linux-2.6/lib/radix-tree.c
> ===================================================================
> --- linux-2.6.orig/lib/radix-tree.c 2010-08-18 00:01:19.000000000 +1000
> +++ linux-2.6/lib/radix-tree.c 2010-08-18 00:47:55.000000000 +1000
> @@ -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;
> @@ -263,7 +273,7 @@ static int radix_tree_extend(struct radi
> 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++) {
> @@ -274,7 +284,7 @@ static int radix_tree_extend(struct radi
> 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);
> @@ -307,7 +317,7 @@ int radix_tree_insert(struct radix_tree_
> 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;
> @@ -323,8 +333,7 @@ int radix_tree_insert(struct radix_tree_
> 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 */
> @@ -372,7 +381,7 @@ static void *radix_tree_lookup_element(s
> 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))
> @@ -391,7 +400,7 @@ static void *radix_tree_lookup_element(s
> height--;
> } while (height > 0);
>
> - return is_slot ? (void *)slot:node;
> + return is_slot ? (void *)slot : indirect_to_ptr(node);
> }
>
> /**
> @@ -453,7 +462,7 @@ void *radix_tree_tag_set(struct radix_tr
> 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) {
> @@ -507,7 +516,7 @@ void *radix_tree_tag_clear(struct radix_
>
> 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;
> @@ -577,7 +586,7 @@ int radix_tree_tag_get(struct radix_tree
>
> 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))
> @@ -651,7 +660,7 @@ unsigned long radix_tree_range_tag_if_ta
> }
>
> shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
> - slot = radix_tree_indirect_to_ptr(root->rnode);
> + slot = indirect_to_ptr(root->rnode);
>
> for (;;) {
> int offset;
> @@ -861,7 +870,7 @@ radix_tree_gang_lookup(struct radix_tree
> results[0] = node;
> return 1;
> }
> - node = radix_tree_indirect_to_ptr(node);
> + node = indirect_to_ptr(node);
>
> max_index = radix_tree_maxindex(node->height);
>
> @@ -880,7 +889,8 @@ radix_tree_gang_lookup(struct radix_tree
> 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;
> @@ -929,7 +939,7 @@ radix_tree_gang_lookup_slot(struct radix
> 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);
>
> @@ -1054,7 +1064,7 @@ radix_tree_gang_lookup_tag(struct radix_
> results[0] = node;
> return 1;
> }
> - node = radix_tree_indirect_to_ptr(node);
> + node = indirect_to_ptr(node);
>
> max_index = radix_tree_maxindex(node->height);
>
> @@ -1073,7 +1083,8 @@ radix_tree_gang_lookup_tag(struct radix_
> 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;
> @@ -1123,7 +1134,7 @@ radix_tree_gang_lookup_tag_slot(struct r
> 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);
>
> @@ -1159,7 +1170,7 @@ static inline void radix_tree_shrink(str
> 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
> @@ -1172,16 +1183,39 @@ static inline void radix_tree_shrink(str
>
> /*
> * 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);
> }
> }
> @@ -1218,7 +1252,7 @@ void *radix_tree_delete(struct radix_tre
> 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;
> @@ -1260,8 +1294,7 @@ void *radix_tree_delete(struct radix_tre
> 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;
> }
> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c 2010-08-18 00:14:14.000000000 +1000
> +++ linux-2.6/mm/filemap.c 2010-08-18 01:07:20.000000000 +1000
> @@ -631,7 +631,9 @@ 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))
> @@ -647,6 +649,7 @@ repeat:
> goto repeat;
> }
> }
> +out:
> rcu_read_unlock();
>
> return page;
> @@ -764,12 +767,11 @@ 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;
What does the line above do?
> goto restart;
> + }
>
> if (!page_cache_get_speculative(page))
> goto repeat;
> @@ -817,11 +819,7 @@ 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)
> @@ -874,11 +872,7 @@ 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))
>
I just verified that your patch fixes the issue. Thanks for doing this.
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