Re: [PATCH] Fixed a mismatch between the users of radix_tree and the implementation.

From: Salman Qazi
Date: Mon Aug 30 2010 - 19:27:52 EST

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On Tue, Aug 17, 2010 at 2:28 PM, Salman Qazi <sqazi@xxxxxxxxxx> wrote:
>
> 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.

Are we going to proceed with this?
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