[RFC PATCH 06/17] btrfs: priority alloc: introduce three macros to mark block group status

[Su Yue <suy.fnst@cn.fujitsu.com>]

In origin design, the order of space_info::block_groups is changed only
when add/remove block group(s).

In design of priority aware allocator, block groups may be moved from
one priority tree to another one. What the operation is usually that
1)  lock block_group
    down_write first_tree
    down_write second_tree
    do unlink
    ....
    do link
    up_write second_tree
    up_write first_tree
    unlock blcok_group
However, the expected order in find_free_extent() is
2)  down_read tree
    lock block_group
    ...
    unlock block_group
    up_read tree

Obviously, orders of operation 1 and operation 2 are on the contrary
and will cause dead lock.

The ugly method is to introduce special status to mark block group
status.
Then:

1) After priority changed:
    lock block_group
    mark block_group to be updated
    unlock blcok_group

    down_write first_tree
    down_write second_tree

    lock block_group
    check block group should be moved
    do unlink
    ....
    do link
    unlock blcok_group
    up_write second_tree
    up_write first_tree

2) find_free_extent():
    down_read tree
    lock block_group
    check the block group is not in special status
    ...
    unlock block_group
    up_read tree

This patch introduce three macros to represents block group is removing
/need to updated / busy.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
---
 fs/btrfs/extent-tree.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index d63078930a1e..5bae757786dc 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -31,6 +31,15 @@
 
 #undef SCRAMBLE_DELAYED_REFS
 
+/* The block group is used by find_free_extent() */
+#define PRIORITY_BG_BUSY -1
+
+/* The block group is in remove or removed */
+#define PRIORITY_BG_DELETED -2
+
+/* The block group' priority needs to be updated */
+#define PRIORITY_BG_UPDATING -3
+
 /*
  * control flags for do_chunk_alloc's force field
  * CHUNK_ALLOC_NO_FORCE means to only allocate a chunk
-- 
2.19.1