Re: [PATCH] Btrfs: fix race in run_clustered refs

[Josef Bacik <jbacik@fusionio.com>]

On Wed, Aug 08, 2012 at 01:49:06PM -0600, Arne Jansen wrote:
> run_clustered_refs runs all delayed refs for one head one by one. During
> the runs, the delayed_refs->lock is released. In this window, the ref_mod
> from the head does not match the sum of all refs below the head. When
> btrfs_lookup_extent_info is run in this window, it gives inconsistent
> results.
> The qgroups patch added code to put delayed refs back, thus opening this
> window very wide.
> This patch assures that head->ref_mod always matches the queued refs, but
> a window still remains where on-disk refs + delayed_refs miss the ref
> currently being run.
> 
> Signed-off-by: Arne Jansen <sensille@gmx.net>
> ---
>  fs/btrfs/extent-tree.c |   17 +++++++++++++++++
>  1 files changed, 17 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index e66dc9a..60d175a 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2318,6 +2318,23 @@ static noinline int run_clustered_refs(struct btrfs_trans_handle *trans,
>  		ref->in_tree = 0;
>  		rb_erase(&ref->rb_node, &delayed_refs->root);
>  		delayed_refs->num_entries--;
> +		if (locked_ref) {
> +			/*
> +			 * when we play the delayed ref, also correct the
> +			 * ref_mod on head
> +			 */
> +			switch (ref->action) {
> +			case BTRFS_ADD_DELAYED_REF:
> +			case BTRFS_ADD_DELAYED_EXTENT:
> +				locked_ref->node.ref_mod -= ref->ref_mod;
> +				break;
> +			case BTRFS_DROP_DELAYED_REF:
> +				locked_ref->node.ref_mod += ref->ref_mod;
> +				break;
> +			default:
> +				WARN_ON(1);
> +			}
> +		}
>  		spin_unlock(&delayed_refs->lock);
>  
>  		ret = run_one_delayed_ref(trans, root, ref, extent_op,

btrfs_lookup_extent_info takes the mutex on the head before it looks at it's
ref_mod, so it should always be consistent.  Maybe somebody else is messing with
refs and not doing the same thing?  If that's the case we should fix them by
doing the same thing, this isn't a fix.  Thanks,

Josef