[PATCH 4/5] btrfs: don't report success for a skipped log sync

[Josef Bacik]

When a commit of the log root tree gives up, because the transaction
needs a full commit or writing the log failed, btrfs_sync_log() still
advances log_transid_committed and hands the result to the contexts on
the list. Unlike a successful commit it does not advance log_transid, so
the log transaction it just retired is still the one new syncs join.

A sync that gets to the log root tree after that adds its context to
the emptied list, sees log_transid_committed >= its log_transid and
returns its log_ret, which is still 0:

  fsync A (subvol 1)           rename       fsync B (subvol 2)

                                            btrfs_need_log_full_commit()
                                              -> false
                               btrfs_set_log_full_commit()
  btrfs_need_log_full_commit()
    -> true
  btrfs_remove_all_log_ctxs()
  log_transid_committed = N
  log_transid stays N
                                            log_transid = N, log_ret = 0
                                            log_transid_committed >= N
                                            return 0

btrfs_sync_file() then ends the transaction without committing it, and a
crash before A's transaction commit loses what B was told is durable.
This was reproduced with delays added to btrfs_sync_log().

Fix by advancing log_transid when a commit of the log root tree gives
up, so a later sync joins the next log transaction and either commits it
or gives up itself. The check for an already committed log transaction
can then no longer be true, so assert that it isn't, and still force a
full commit if it is.

Fixes: d1433debe7f4 ("Btrfs: just wait or commit our own log sub-transaction")
CC: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 fs/btrfs/tree-log.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 1d9dfb63fc5a..b542b1d97989 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3437,11 +3437,20 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 		goto out;
 	}
 
-	if (log_root_tree->log_transid_committed >= root_log_ctx.log_transid) {
+	/*
+	 * A commit of the log root tree moves log_transid on even when it
+	 * gives up, so the log transaction we just joined can't be done.
+	 */
+	if (unlikely(log_root_tree->log_transid_committed >=
+		     root_log_ctx.log_transid)) {
+		ASSERT(0, "log_transid_committed=%d log_transid=%d",
+		       log_root_tree->log_transid_committed,
+		       root_log_ctx.log_transid);
 		blk_finish_plug(&plug);
 		list_del_init(&root_log_ctx.list);
+		btrfs_wait_tree_log_extents(log, mark);
 		mutex_unlock(&log_root_tree->log_mutex);
-		ret = root_log_ctx.log_ret;
+		ret = BTRFS_LOG_FORCE_COMMIT;
 		trace_btrfs_sync_log_exit(trans, root, ctx, ret);
 		goto out;
 	}
@@ -3575,6 +3584,13 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	mutex_lock(&log_root_tree->log_mutex);
 	btrfs_remove_all_log_ctxs(log_root_tree, index2, ret);
 
+	/*
+	 * If the commit gave up, log_transid still names the log transaction
+	 * we are retiring. Move it on, so that nobody joins a log transaction
+	 * whose result has already been handed out.
+	 */
+	if (log_root_tree->log_transid == root_log_ctx.log_transid)
+		log_root_tree->log_transid++;
 	log_root_tree->log_transid_committed++;
 	log_root_tree->log_commit[index2] = false;
 	mutex_unlock(&log_root_tree->log_mutex);

-- 
2.55.0