Hi folks, We recently found a zero-day log recovery issue where overlapping log transactions used the wrong LSN for recovery operations despite being replayed in the correct commit record order. The issue is that the log recovery code uses the LSN of the start record of the log transaction for ordering and metadata stamping, while the actual order of transaction replay is determined by the commit record. Hence if we pipeline CIL commits we can end up with overlapping transactions in the log like: Start A .. Start C .. Start B .... Commit A .. Commit B .. Commit C The issue is that the "start B" lsn is later than the "start C" lsn. When the same metadata block is modified in both transaction B and C, writeback from "commit B" will correctly stamp "start B" into the metadata. However, when "commit C" runs, it will see the LSN in that metadata block is "start B", which is *more recent than "Start C" and so will, incorrectly, fail to recover that change into the metadata block. This results in silent metadata corruption, which can then be exposed by future recovery operations failing, runtime inconsistencies causing shutdowns and/or xfs_scrub/xfs_repair check failures. We could fix log recovery to avoid this problem, but there's a runtime problem as well: the AIL is ordered by start record LSN. We cannot order the AIL by commit LSN as we cannot allow the tail of the log to overwrite -any- of the log transaction until the entire transaction has been written. As the lowest LSN of the items in the AIL defines the current log tail, the same metadata writeback ordering issues apply as with log recovery. In this case, we run the callbacks for commit B first, which place all the items at the head of the log at "start B". Then we run callbacks for "commit C", which then do not insert at the head - they get inserted before "start B". If the item was modified in both B and C, then it moves *backwards* in the AIL and this screws up all manner of things that assume relogging can only move objects forwards in the log. One of these things it can screw up is the tail lsn of the log. Nothing good comes from this... Because we have both runtime and journal-based ordering requirements for the start_lsn, we have multiple places where there is an implicit assumption that transaction start records are strictly ordered. Rather than play whack-a-mole with such assumptions, and to avoid the eternal "are you running a fixed kernel" question, it's better just to strictly order the start records in the same way we strictly order the commit records. This patch series takes the mechanisms of the strict commit record ordering and utilises them for strict start record ordering. It builds upon the shutdown rework patchset to guarantee that the CIL context structure will not get freed from under it by a racing shutdown, and so moves the LSN recording for ordering up into a callback from xlog_write() once we have a guaranteed iclog write location. This means we have one mechanism for both start and commit record ordering, and they both work in exactly the same way. Cheers, Dave.
