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Message-ID: <20100820080207.GC3777@amd>
Date: Fri, 20 Aug 2010 18:02:07 +1000
From: Nick Piggin <npiggin@...nel.dk>
To: Dave Chinner <david@...morbit.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
jack@...e.cz, npiggin@...nel.dk
Subject: Re: [PATCH 2/2] radix-tree: radix_tree_range_tag_if_tagged() can
set incorrect tags
On Fri, Aug 20, 2010 at 03:22:07PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@...hat.com>
>
> Commit ebf8aa44beed48cd17893a83d92a4403e5f9d9e2 ("radix-tree:
> omplement function radix_tree_range_tag_if_tagged") does not safely
> set tags on on intermediate tree nodes. The code walks down the tree
> setting tags before it has fully resolved the path to the leaf under
> the assumption there will be a leaf slot with the tag set in the
> range it is searching.
>
> Unfortunately, this is not a valid assumption - we can abort after
> setting a tag on an intermediate node if we overrun the number of
> tags we are allowed to set in a batch, or stop scanning because we
> we have passed the last scan index before we reach a leaf slot with
> the tag we are searching for set.
>
> As a result, we can leave the function with tags set on intemediate
> nodes which can be tripped over later by tag-based lookups. The
> result of these stale tags is that lookup may end prematurely or
> livelock because the lookup cannot make progress.
>
> The fix for the problem involves reocrding the traversal path we
> take to the leaf nodes, and only propagating the tags back up the
> tree once the tag is set in the leaf node slot. We are already
> recording the path for efficient traversal, so there is no
> additional overhead to do the intermediately node tag setting in
> this manner.
>
> This fixes a radix tree lookup livelock triggered by the new
> writeback sync livelock avoidance code introduced in commit
> f446daaea9d4a420d16c606f755f3689dcb2d0ce ("mm: implement writeback
> livelock avoidance using page tagging").
It seems OK to me, thanks.
>
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> ---
> lib/radix-tree.c | 57 +++++++++++++++++++++++++++++++++++++++++------------
> 1 files changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index 1014171..e0ee8cb 100644
> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -625,6 +625,13 @@ EXPORT_SYMBOL(radix_tree_tag_get);
> * also settag. The function stops either after tagging nr_to_tag items or
> * after reaching last_index.
> *
> + * The tags must be set from the leaf level only and propagated back up the
> + * path to the root. We must do this so that we resolve the full path before
> + * setting any tags on intermediate nodes. If we set tags as we descend, then
> + * we can get to the leaf node and find that the index that has the iftag
> + * set is outside the range we are scanning. This reults in dangling tags and
> + * can lead to problems with later tag operations (e.g. livelocks on lookups).
> + *
> * The function returns number of leaves where the tag was set and sets
> * *first_indexp to the first unscanned index.
> */
> @@ -633,9 +640,13 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
> unsigned long nr_to_tag,
> unsigned int iftag, unsigned int settag)
> {
> - unsigned int height = root->height, shift;
> - unsigned long tagged = 0, index = *first_indexp;
> - struct radix_tree_node *open_slots[height], *slot;
> + unsigned int height = root->height;
> + struct radix_tree_path path[height];
> + struct radix_tree_path *pathp = path;
> + struct radix_tree_node *slot;
> + unsigned int shift;
> + unsigned long tagged = 0;
> + unsigned long index = *first_indexp;
>
> last_index = min(last_index, radix_tree_maxindex(height));
> if (index > last_index)
> @@ -655,6 +666,13 @@ 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);
>
> + /*
> + * we fill the path from (root->height - 2) to 0, leaving the index at
> + * (root->height - 1) as a terminator. Zero the node in the terminator
> + * so that we can use this to end walk loops back up the path.
> + */
> + path[height - 1].node = NULL;
> +
> for (;;) {
> int offset;
>
> @@ -663,17 +681,30 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
> goto next;
> if (!tag_get(slot, iftag, offset))
> goto next;
> + if (height > 1) {
> + /* Go down one level */
> + height--;
> + shift -= RADIX_TREE_MAP_SHIFT;
> + path[height - 1].node = slot;
> + path[height - 1].offset = offset;
> + slot = slot->slots[offset];
> + continue;
> + }
> +
> + /* tag the leaf */
> + tagged++;
> tag_set(slot, settag, offset);
> - if (height == 1) {
> - tagged++;
> - goto next;
> +
> + /* walk back up the path tagging interior nodes */
> + pathp = &path[0];
> + while (pathp->node) {
> + /* stop if we find a node with the tag already set */
> + if (tag_get(pathp->node, settag, pathp->offset))
> + break;
> + tag_set(pathp->node, settag, pathp->offset);
> + pathp++;
> }
> - /* Go down one level */
> - height--;
> - shift -= RADIX_TREE_MAP_SHIFT;
> - open_slots[height] = slot;
> - slot = slot->slots[offset];
> - continue;
> +
> next:
> /* Go to next item at level determined by 'shift' */
> index = ((index >> shift) + 1) << shift;
> @@ -687,7 +718,7 @@ next:
> * last_index is guaranteed to be in the tree, what
> * we do below cannot wander astray.
> */
> - slot = open_slots[height];
> + slot = path[height - 1].node;
> height++;
> shift += RADIX_TREE_MAP_SHIFT;
> }
> --
> 1.7.1
--
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