1 files changed, 10 insertions, 2 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index c43bbc7f623e..c3cfdfd8de9b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1981,16 +1981,24 @@ static void btrfs_cleanup_pending_block_groups(struct btrfs_trans_handle *trans)
 static inline int btrfs_start_delalloc_flush(struct btrfs_fs_info *fs_info)
 {
 	/*
-	 * We use writeback_inodes_sb here because if we used
+	 * We use try_to_writeback_inodes_sb() here because if we used
 	 * btrfs_start_delalloc_roots we would deadlock with fs freeze.
 	 * Currently are holding the fs freeze lock, if we do an async flush
 	 * we'll do btrfs_join_transaction() and deadlock because we need to
 	 * wait for the fs freeze lock.  Using the direct flushing we benefit
 	 * from already being in a transaction and our join_transaction doesn't
 	 * have to re-take the fs freeze lock.
+	 *
+	 * Note that try_to_writeback_inodes_sb() will only trigger writeback
+	 * if it can read lock sb->s_umount. It will always be able to lock it,
+	 * except when the filesystem is being unmounted or being frozen, but in
+	 * those cases sync_filesystem() is called, which results in calling
+	 * writeback_inodes_sb() while holding a write lock on sb->s_umount.
+	 * Note that we don't call writeback_inodes_sb() directly, because it
+	 * will emit a warning if sb->s_umount is not locked.
 	 */
 	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT))
-		writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);
+		try_to_writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);
 	return 0;
 }