[PATCH] btrfs: zoned: move log tree node allocation out of log_root_tree->log_mutex

[Naohiro Aota]

Commit 6e37d2459941 ("btrfs: zoned: fix deadlock on log sync") pointed out
a deadlock warning and removed
mutex_{lock,unlock}(&fs_info->tree_root->log_mutex). While it looks like it
always cause a deadlock, we didn't see actual deadlock in fstests runs. The
reason is log_root_tree->log_mutex != fs_info->tree_root->log_mutex, not
taking the same lock. So, the warning was actually a false-positive.

Since btrfs_alloc_log_tree_node() is protected only by
fs_info->tree_root->log_mutex, we can (and should) move the code out of the
lock scope of log_root_tree->log_mutex and silence the warning.

Cc: Filipe Manana <fdman...@suse.com>
Cc: Johannes Thumshirn <johannes.thumsh...@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.a...@wdc.com>
---
 fs/btrfs/tree-log.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 92a368627791..72c4b66ed516 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3165,20 +3165,22 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
         */
        mutex_unlock(&root->log_mutex);
 
-       btrfs_init_log_ctx(&root_log_ctx, NULL);
-
-       mutex_lock(&log_root_tree->log_mutex);
-
        if (btrfs_is_zoned(fs_info)) {
+               mutex_lock(&fs_info->tree_root->log_mutex);
                if (!log_root_tree->node) {
                        ret = btrfs_alloc_log_tree_node(trans, log_root_tree);
                        if (ret) {
-                               mutex_unlock(&log_root_tree->log_mutex);
+                               mutex_unlock(&fs_info->tree_log_mutex);
                                goto out;
                        }
                }
+               mutex_unlock(&fs_info->tree_root->log_mutex);
        }
 
+       btrfs_init_log_ctx(&root_log_ctx, NULL);
+
+       mutex_lock(&log_root_tree->log_mutex);
+
        index2 = log_root_tree->log_transid % 2;
        list_add_tail(&root_log_ctx.list, &log_root_tree->log_ctxs[index2]);
        root_log_ctx.log_transid = log_root_tree->log_transid;
-- 
2.31.0