On Tue, Aug 17, 2010 at 8:23 AM, Nick Piggin <npiggin@kernel.dk> wrote:
quoted text
> 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?


quoted text
>                        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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