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  1. ZABBIX BUGS AND ISSUES
  2. ZBX-16927

Pollers stuck waiting for response without timeout

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Details

    • Problem report
    • Status: Need info
    • Trivial
    • Resolution: Unresolved
    • 4.2.8
    • None
    • Proxy (P), Server (S)
    • Ubuntu 18.04
    • Team A

    Description

      It acted the same in version 3.4 too. Tried to upgrade but it did not help.

      Steps to reproduce:

      1. Setup few SNMPv3 hosts
      2. After some time (few to several hours) notice all pollers (both unreachable and regular ones) are busy. Hosts are supposedly down.

      Result:

      see "Annotation 2019-11-15 133726.jpg"

      see "graph_pollers_busy.jpg"

      Please bear with me...

      I checked few things and this is what I found out.

      First I went to see PS output and noticed that poller and unreachable pollers descriptions do not update at all. See 'ps-pollers-getting-values.jpg'.

      Tried strace main zabbix process (with child processes), but there was no action there too. See 'strace-main-zabbix-server-with-child-processes-stuck.jpg'

      Then went to strace the pollers. All of them were stuck on select call (tried waiting for a bit) without timeout reading from descriptor 10 - a UDP socket. See 'strace-poller-process-stuck-on-select.jpg' and 'lsof-udp-fd-10.jpg'

      What's it doing? Here's a backtrace from gdb - see "gdb-poller-process-bt.jpg"

      In zabbix sources it said NETSNMP has its own timeout values, I checked there and saw this piece of code (version 5.4.4) - notice / block without timeout / comment:

      int
      snmp_synch_response_cb(netsnmp_session * ss,
                             netsnmp_pdu *pdu,
                             netsnmp_pdu **response, snmp_callback pcb)
      {
          struct synch_state lstate, *state;
          snmp_callback   cbsav;
          void           *cbmagsav;
          int             numfds, count;
          fd_set          fdset;
          struct timeval  timeout, *tvp;
          int             block;
      
          memset((void *) &lstate, 0, sizeof(lstate));
          state = &lstate;
          cbsav = ss->callback;
          cbmagsav = ss->callback_magic;
          ss->callback = pcb;
          ss->callback_magic = (void *) state;
      
          if ((state->reqid = snmp_send(ss, pdu)) == 0) {
              snmp_free_pdu(pdu);
              state->status = STAT_ERROR;
          } else
              state->waiting = 1;
      
          while (state->waiting) {
              numfds = 0;
              FD_ZERO(&fdset);
              block = NETSNMP_SNMPBLOCK;
              tvp = &timeout;
              timerclear(tvp);
              snmp_select_info(&numfds, &fdset, tvp, &block);
              if (block == 1)
                  tvp = NULL;         /* block without timeout */
              count = select(numfds, &fdset, 0, 0, tvp);
              if (count > 0) {
                  snmp_read(&fdset);
              } else {
                  switch (count) {
                  case 0:
                      snmp_timeout();
                      break;
                  case -1:
                      if (errno == EINTR) {
                          continue;
                      } else {
                          snmp_errno = SNMPERR_GENERR;    /*MTCRITICAL_RESOURCE */
                          /*
                           * CAUTION! if another thread closed the socket(s)
                           * waited on here, the session structure was freed.
                           * It would be nice, but we can't rely on the pointer.
                           * ss->s_snmp_errno = SNMPERR_GENERR;
                           * ss->s_errno = errno;
                           */
                          snmp_set_detail(strerror(errno));
                      }
                      /*
                       * FALLTHRU 
                       */
                  default:
                      state->status = STAT_ERROR;
                      state->waiting = 0;
                  }
              }
      
              if ( ss->flags & SNMP_FLAGS_RESP_CALLBACK ) {
                  void (*cb)(void);
                  cb = ss->myvoid;
                  cb();        /* Used to invoke 'netsnmp_check_outstanding_agent_requests();'
                                  on internal AgentX queries.  */
              }
          }
          *response = state->pdu;
          ss->callback = cbsav;
          ss->callback_magic = cbmagsav;
          return state->status;
      }
      

      So all my pollers seem to be stuck waiting forever for a response from UDP socket.

      After server restart it goes back to normal.

       

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            zabbix.dev Zabbix Development Team
            gregolsky Grzegorz Lachowski
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              Created:
              Updated: