Re: [PATCH] Btrfs: fix a deadlock on chunk mutex

[Josef Bacik <jbacik@fusionio.com>]

On Wed, Dec 12, 2012 at 06:52:37PM -0700, Liu Bo wrote:
> An user reported that he has hit an annoying deadlock while playing with
> ceph based on btrfs.
> 
> Current updating device tree requires space from METADATA chunk,
> so we -may- need to do a recursive chunk allocation when adding/updating
> dev extent, that is where the deadlock comes from.
> 
> If we use SYSTEM metadata to update device tree, we can avoid the recursive
> stuff.
> 

This is going to cause us to allocate much more system chunks than we used to
which could land us in trouble.  Instead let's just keep us from re-entering if
we're already allocating a chunk.  We do the chunk allocation when we don't have
enough space for a cluster, but we'll likely have plenty of space to make an
allocation.  Can you give this patch a try Jim and see if it fixes your problem?
Thanks,

Josef


diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index e152809..59df5e7 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3564,6 +3564,10 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
 	int wait_for_alloc = 0;
 	int ret = 0;
 
+	/* Don't re-enter if we're already allocating a chunk */
+	if (trans->allocating_chunk)
+		return -ENOSPC;
+
 	space_info = __find_space_info(extent_root->fs_info, flags);
 	if (!space_info) {
 		ret = update_space_info(extent_root->fs_info, flags,
@@ -3606,6 +3610,8 @@ again:
 		goto again;
 	}
 
+	trans->allocating_chunk = true;
+
 	/*
 	 * If we have mixed data/metadata chunks we want to make sure we keep
 	 * allocating mixed chunks instead of individual chunks.
@@ -3632,6 +3638,7 @@ again:
 	check_system_chunk(trans, extent_root, flags);
 
 	ret = btrfs_alloc_chunk(trans, extent_root, flags);
+	trans->allocating_chunk = false;
 	if (ret < 0 && ret != -ENOSPC)
 		goto out;
 
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index e6509b9..47ad8be 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -388,6 +388,7 @@ again:
 	h->qgroup_reserved = qgroup_reserved;
 	h->delayed_ref_elem.seq = 0;
 	h->type = type;
+	h->allocating_chunk = false;
 	INIT_LIST_HEAD(&h->qgroup_ref_list);
 	INIT_LIST_HEAD(&h->new_bgs);
 
diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index 0e8aa1e..69700f7 100644
--- a/fs/btrfs/transaction.h
+++ b/fs/btrfs/transaction.h
@@ -68,6 +68,7 @@ struct btrfs_trans_handle {
 	struct btrfs_block_rsv *orig_rsv;
 	short aborted;
 	short adding_csums;
+	bool allocating_chunk;
 	enum btrfs_trans_type type;
 	/*
 	 * this root is only needed to validate that the root passed to