Btrfs: preserve commit_root for async caching

The async block group caching code uses the commit_root pointer
to get a stable version of the extent allocation tree for scanning.
This copy of the tree root isn't going to change and it significantly
reduces the complexity of the scanning code.

During a commit, we have a loop where we update the extent allocation
tree root.  We need to loop because updating the root pointer in
the tree of tree roots may allocate blocks which may change the
extent allocation tree.

Right now the commit_root pointer is changed inside this loop.  It
is more correct to change the commit_root pointer only after all the
looping is done.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index fadf69a2764b..2fe21fa74913 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -267,7 +267,7 @@ static int caching_kthread(void *data)
         last = max_t(u64, block_group->key.objectid, BTRFS_SUPER_INFO_OFFSET);
 again:
         /* need to make sure the commit_root doesn't disappear */
-        down_read(&fs_info->extent_root->commit_root_sem);
+        down_read(&fs_info->extent_commit_sem);
 
         /*
          * We don't want to deadlock with somebody trying to allocate a new
@@ -304,7 +304,7 @@ again:
 
                         if (need_resched()) {
                                 btrfs_release_path(fs_info->extent_root, path);
-                                up_read(&fs_info->extent_root->commit_root_sem);
+                                up_read(&fs_info->extent_commit_sem);
                                 cond_resched();
                                 goto again;
                         }
@@ -345,7 +345,7 @@ next:
 
 err:
         btrfs_free_path(path);
-        up_read(&fs_info->extent_root->commit_root_sem);
+        up_read(&fs_info->extent_commit_sem);
         atomic_dec(&block_group->space_info->caching_threads);
         wake_up(&block_group->caching_q);