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  1. Core Server
  2. SERVER-35217

killSessions command attempts to kill a transaction while holding SessionCatalog::_mutex, which leads to deadlock

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    • Fully Compatible
    • ALL
    • v4.0
    • Repl 2018-06-18

    Description

      Attempting to acquire a LockManager lock while holding a mutex is prone to deadlock. The SessionCatalog::scanSessions() function locks the session catalog and executes an arbitrary function on any matching sessions. For the "killSessions" command, this means that while holding the SessionCatalog::_mutex, it calls CursorManager::killAllCursorsForTransaction(), which for a find cursor attempts to acquire the collection lock in CursorManager::withCursorManager(). Since the killAllExpiredTransactions() function also calls SessionCatalog::scanSessions(), it isn't possible to reap expired transactions while the "killSessions" command is running. It similarly isn't possible to commit or abort a transaction while the "killSessions" command is running because SessionCatalog::checkOutSession() must acquire the SessionCatalog::_mutex.

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              tess.avitabile@mongodb.com Tess Avitabile (Inactive)
              max.hirschhorn@mongodb.com Max Hirschhorn
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