refcounts have a generic implementation and an asm optimized one. The
generic version has extra debugging to make sure that once a refcount
goes to zero, refcount_inc won't increase it.
The btrfs delayed inode code wasn't expecting this, and we're tripping
over the warnings when the generic refcounts are used. We ended up with
this race:
Process A Process B
btrfs_get_delayed_node()
spin_lock(root->inode_lock)
radix_tree_lookup()
__btrfs_release_delayed_node()
refcount_dec_and_test(&delayed_node->refs)
our refcount is now zero
refcount_add(2) <---
warning here, refcount
unchanged
spin_lock(root->inode_lock)
radix_tree_delete()
With the generic refcounts, we actually warn again when process B above
tries to release his refcount because refcount_add() turned into a
no-op.
We saw this in production on older kernels without the asm optimized
refcounts.
The fix used here is to use refcount_inc_not_zero() to detect when the
object is in the middle of being freed and return NULL. This is almost
always the right answer anyway, since we usually end up pitching the
delayed_node if it didn't have fresh data in it.
This also changes __btrfs_release_delayed_node() to remove the extra
check for zero refcounts before radix tree deletion.
btrfs_get_delayed_node() was the only path that was allowing refcounts
to go from zero to one.
Signed-off-by: Chris Mason <c...@fb.com>
Fixes: 6de5f18e7b0da
cc: <sta...@vger.kernel.org> #4.12+
---
fs/btrfs/delayed-inode.c | 45 ++++++++++++++++++++++++++++++++++-----------
1 file changed, 34 insertions(+), 11 deletions(-)
diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
index 5d73f79..84c54af 100644
--- a/fs/btrfs/delayed-inode.c
+++ b/fs/btrfs/delayed-inode.c
@@ -87,6 +87,7 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
spin_lock(&root->inode_lock);
node = radix_tree_lookup(&root->delayed_nodes_tree, ino);
+
if (node) {
if (btrfs_inode->delayed_node) {
refcount_inc(&node->refs); /* can be accessed */
@@ -94,9 +95,30 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
spin_unlock(&root->inode_lock);
return node;
}
- btrfs_inode->delayed_node = node;
- /* can be accessed and cached in the inode */
- refcount_add(2, &node->refs);
+
+ /* it's possible that we're racing into the middle of
+ * removing this node from the radix tree. In this case,
+ * the refcount was zero and it should never go back
+ * to one. Just return NULL like it was never in the radix
+ * at all; our release function is in the process of removing
+ * it.
+ *
+ * Some implementations of refcount_inc refuse to
+ * bump the refcount once it has hit zero. If we don't do
+ * this dance here, refcount_inc() may decide to
+ * just WARN_ONCE() instead of actually bumping the refcount.
+ *
+ * If this node is properly in the radix, we want to
+ * bump the refcount twice, once for the inode
+ * and once for this get operation.
+ */
+ if (refcount_inc_not_zero(&node->refs)) {
+ refcount_inc(&node->refs);
+ btrfs_inode->delayed_node = node;
+ } else {
+ node = NULL;
+ }
+
spin_unlock(&root->inode_lock);
return node;
}
@@ -254,17 +276,18 @@ static void __btrfs_release_delayed_node(
mutex_unlock(&delayed_node->mutex);
if (refcount_dec_and_test(&delayed_node->refs)) {
- bool free = false;
struct btrfs_root *root = delayed_node->root;
+
spin_lock(&root->inode_lock);
- if (refcount_read(&delayed_node->refs) == 0) {
- radix_tree_delete(&root->delayed_nodes_tree,
- delayed_node->inode_id);
- free = true;
- }
+ /*
+ * once our refcount goes to zero, nobody is allowed to
+ * bump it back up. We can delete it now
+ */
+ ASSERT(refcount_read(&delayed_node->refs) == 0);
+ radix_tree_delete(&root->delayed_nodes_tree,
+ delayed_node->inode_id);
spin_unlock(&root->inode_lock);
- if (free)
- kmem_cache_free(delayed_node_cache, delayed_node);
+ kmem_cache_free(delayed_node_cache, delayed_node);
}
}
--
2.9.5
--
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