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  1. WiredTiger
  2. WT-11469

Missing read barrier in __wt_txn_upd_visible_all after reading the prepared state

    • Storage Engines
    • 8
    • BermudaTriangle- 2023-09-05, TheMoon-StorEng - 2023-09-19, NachoCheese - 2023-10-03, Joker - StorEng - 2023-10-17, Asparagus-StorEng - 2023-10-31, c(3x10^8)-StorEng - 2023-11-14, 2023-11-28 - Anthill Tiger, 2023-12-12 - Heisenbug, 2024-01-09 - I Grew Tired

      static inline bool
      __wt_txn_upd_visible_all(WT_SESSION_IMPL *session, WT_UPDATE *upd)
          if (upd->prepare_state == WT_PREPARE_LOCKED || upd->prepare_state == WT_PREPARE_INPROGRESS)
              return (false);
           * This function is used to determine when an update is obsolete: that should take into account
           * the durable timestamp which is greater than or equal to the start timestamp.
          return (__wt_txn_visible_all(session, upd->txnid, upd->durable_ts));

      Here we are not ensuring we always read the prepared state before reading the durable_ts by inserting a read barrier. It is possible the compiler and cpu reorders the read and reads a very old value for durable_ts before reading the prepared state. This can leads to data corruption by prematurely discarding the updates.

      We should do a proper review of the code and fix it in other places to ensure we always read the prepared state before the timestamps.

            clarisse.cheah@mongodb.com Clarisse Cheah
            chenhao.qu@mongodb.com Chenhao Qu
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