ext4: avoid journaling sb update on error if journal is destroying

Presently we always BUG_ON if trying to start a transaction on a journal marked
with JBD2_UNMOUNT, since this should never happen. However, while ltp running
stress tests, it was observed that in case of some error handling paths, it is
possible for update_super_work to start a transaction after the journal is
destroyed eg:

(umount)
ext4_kill_sb
  kill_block_super
    generic_shutdown_super
      sync_filesystem /* commits all txns */
      evict_inodes
        /* might start a new txn */
      ext4_put_super
	flush_work(&sbi->s_sb_upd_work) /* flush the workqueue */
        jbd2_journal_destroy
          journal_kill_thread
            journal->j_flags |= JBD2_UNMOUNT;
          jbd2_journal_commit_transaction
            jbd2_journal_get_descriptor_buffer
              jbd2_journal_bmap
                ext4_journal_bmap
                  ext4_map_blocks
                    ...
                    ext4_inode_error
                      ext4_handle_error
                        schedule_work(&sbi->s_sb_upd_work)

                                               /* work queue kicks in */
                                               update_super_work
                                                 jbd2_journal_start
                                                   start_this_handle
                                                     BUG_ON(journal->j_flags &
                                                            JBD2_UNMOUNT)

Hence, introduce a new mount flag to indicate journal is destroying and only do
a journaled (and deferred) update of sb if this flag is not set. Otherwise, just
fallback to an un-journaled commit.

Further, in the journal destroy path, we have the following sequence:

  1. Set mount flag indicating journal is destroying
  2. force a commit and wait for it
  3. flush pending sb updates

This sequence is important as it ensures that, after this point, there is no sb
update that might be journaled so it is safe to update the sb outside the
journal. (To avoid race discussed in 2d01ddc8)

Also, we don't need a similar check in ext4_grp_locked_error since it is only
called from mballoc and AFAICT it would be always valid to schedule work here.

Fixes: 2d01ddc8 ("ext4: save error info to sb through journal if available")
Reported-by: Mahesh Kumar <maheshkumar657g@gmail.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/9613c465d6ff00cd315602f99283d5f24018c3f7.1742279837.git.ojaswin@linux.ibm.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>