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  1. WiredTiger
  2. WT-944

overflow cache race in format

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    Details

    • Type: Task
    • Status: Closed
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: WT2.2
    • Labels:

      Description

      core dump from 3c153e9162bc57858ee7e4f2b0ae97d2c9b503dc with this CONFIG

      file_type=row
      data_source=file
      cache=5
      compression=none
      leaf_page_max=9
      internal_page_max=9
      ops=200000
      rows=1000
      key_min=200
      threads=8
      insert_pct=0
      

      Here's the stack:

      #0  0x00000000004b5e92 in __ovfl_txnc_skip_search (head=0x803fa3850, 
          addr=0x80279c472, addr_size=8) at ../src/btree/rec_track.c:631
      WT-1  0x00000000004b6287 in __wt_ovfl_txnc_search (page=0x8098c0b40, 
          addr=0x80279c472 "?5\202?\227?*`\210? \201????]?\210?\234\201?\001\022@??\210ư\201???\2113?\210?J\201?\001cfR?\210?V\201??\016??\210?w\202?k7???\210?\"\201?\025\020\031??\210?[\202?\224??\020?\210?[\201?\231&?$?\210?f\204?3b\202\016?\210??\201??J*??\210ͧ\202??d?\237`\210?l\201?\v\214\220?\210?\235\202?????`\210?)\201?\237@H??\210??\204?av?B`\210?\020\201?2??", addr_size=8, 
          store=0x8033f39e8) at ../src/btree/rec_track.c:765
      WT-2  0x00000000004ae54c in __wt_ovfl_read (session=0x802500180, 
          page=0x8098c0b40, unpack=0x7fffff3f9c80, store=0x8033f39e8)
          at ../src/btree/bt_ovfl.c:64
      WT-3  0x00000000004b09c4 in __cell_data_ref (session=0x802500180, 
          page=0x8098c0b40, page_type=7, unpack=0x7fffff3f9c80, store=0x8033f39e8)
          at cell.i:788
      WT-4  0x00000000004b087b in __wt_page_cell_data_ref (session=0x802500180, 
          page=0x8098c0b40, unpack=0x7fffff3f9c80, store=0x8033f39e8) at cell.i:823
      WT-5  0x00000000004af93f in __wt_kv_return (session=0x802500180, cbt=0x8033f3900)
          at ../src/btree/bt_ret.c:107
      WT-6  0x00000000004a6cee in __wt_btcur_search (cbt=0x8033f3900)
          at ../src/btree/bt_cursor.c:246
      WT-7  0x000000000047da3a in __curfile_search (cursor=0x8033f3900)
          at ../src/cursor/cur_file.c:177
      WT-8  0x0000000000407cd2 in row_remove (cursor=0x8033f3900, key=0x7fffff3f9f30, 
          keyno=83, notfoundp=0x7fffff3f9ee4) at ../../../test/format/ops.c:979
       
       
      (gdb) frame 0
      #0  0x00000000004b5e92 in __ovfl_txnc_skip_search (head=0x803fa3850, 
          addr=0x80279c472, addr_size=8) at ../src/btree/rec_track.c:631
      631			len = WT_MIN((*e)->addr_size, addr_size);
      (gdb) p e
      $19 = (WT_OVFL_TXNC **) 0x803fa3850
      (gdb) p *e
      $20 = (WT_OVFL_TXNC *) 0x0
      

      Since the code tests *e == NULL a few lines before the core dump:

      for (i = WT_SKIP_MAXDEPTH - 1, e = &head[i]; i >= 0;) {
              if (*e == NULL) {               /* Empty levels */
                      --i;
                      --e;
                      continue;
              }
              
              /*
               * Return any exact matches: we don't care in what search level
               * we found a match.
               */
              len = WT_MIN((*e)->addr_size, addr_size);
              cmp = memcmp(WT_OVFL_TXNC_ADDR(*e), addr, len);
              if (cmp == 0 && (*e)->addr_size == addr_size)
                      return (*e);
      

      I'm guessing it's a race. The caller is not protected against a race with a thread removing the overflow key from the cache:

      /*
       * WT_CELL_VALUE_OVFL_RM cells: If reconciliation deleted an overflow
       * value, but there was still a reader in the system that might need it,
       * the on-page cell type will have been reset to WT_CELL_VALUE_OVFL_RM
       * and we will be passed a page so we can look-aside into the cache of
       * such values.
       */
      if (unpack->raw == WT_CELL_VALUE_OVFL_RM)
          return (
              __wt_ovfl_txnc_search(page, unpack->data, unpack->size, store));
               
      /*
       * Acquire the overflow lock, and retest the on-page cell's value inside
       * the lock.
       */
      WT_RET(__wt_readlock(session, S2BT(session)->ovfl_lock));
      ret = __wt_cell_type_raw(unpack->cell) == WT_CELL_VALUE_OVFL_RM ?
          __wt_ovfl_txnc_search(page, unpack->data, unpack->size, store) :
          __ovfl_read(session, unpack->data, unpack->size, store);
      WT_TRET(__wt_rwunlock(session, S2BT(session)->ovfl_lock));
      

      But that should be OK – if the on-page cell is set to VALUE_OVFL_RM, it can't be removed until all readers that might access it have left the system?

      This may mean there's a case where we can delete an item from the cache based on the transaction tests in that code, but there's still a reader in the system that needs the item.

      @michaelcahill, can you please review?

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              • Assignee:
                keith.bostic Keith Bostic
                Reporter:
                keith.bostic Keith Bostic
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                • Created:
                  Updated:
                  Resolved: