On Fri, Dec 15, 2017 at 11:58:27AM -0800, Chris Mason wrote:
> 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.
>
Reviewed-by: Liu Bo <bo.li....@oracle.com>
-liubo
> 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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