rlc_am_lte: reduce time holding Tx mutex when processing status PDUs

the patch is a re-implementation of the customer-specific optimization
we did in order to reduce the time the RLC holds the Tx mutex when
processing an incoming status PDU.

The patch makes sure to never operate on a raw mutex but instead
uses the deadlock-avoiding RAII lock.

lib/src/upper/rlc_am_lte.cc

@@ -1168,11 +1168,16 @@ void rlc_am_lte::rlc_am_lte_tx::handle_control_pdu(uint8_t* payload, uint32_t no
     return;
   }
 
-  std::unique_lock<std::mutex> lock(mutex);
+  // Local variables for handling Status PDU will be updated with lock
+  rlc_status_pdu_t status     = {};
+  uint32_t         i          = 0;
+  uint32_t         vt_s_local = 0;
+
+  {
+    std::lock_guard<std::mutex> lock(mutex);
 
     logger.debug(payload, nof_bytes, "%s Rx control PDU", RB_NAME);
 
-  rlc_status_pdu_t status;
     rlc_am_read_status_pdu(payload, nof_bytes, &status);
 
     log_rlc_am_status_pdu_to_string(logger.info, "%s Rx Status PDU: %s", &status, RB_NAME);
@@ -1200,16 +1205,18 @@ void rlc_am_lte::rlc_am_lte_tx::handle_control_pdu(uint8_t* payload, uint32_t no
       retx_queue.clear();
     }
 
-  // Handle ACKs and NACKs
-  bool     update_vt_a = true;
-  uint32_t i           = vt_a;
+    i          = vt_a;
+    vt_s_local = vt_s;
+  }
 
-  while (TX_MOD_BASE(i) < TX_MOD_BASE(status.ack_sn) && TX_MOD_BASE(i) < TX_MOD_BASE(vt_s)) {
+  bool update_vt_a = true;
+  while (TX_MOD_BASE(i) < TX_MOD_BASE(status.ack_sn) && TX_MOD_BASE(i) < TX_MOD_BASE(vt_s_local)) {
     bool nack = false;
     for (uint32_t j = 0; j < status.N_nack; j++) {
       if (status.nacks[j].nack_sn == i) {
         nack        = true;
         update_vt_a = false;
+        std::lock_guard<std::mutex> lock(mutex);
         if (tx_window.has_sn(i)) {
           auto& pdu = tx_window[i];
           if (not retx_queue.has_sn(i)) {
@@ -1259,6 +1266,7 @@ void rlc_am_lte::rlc_am_lte_tx::handle_control_pdu(uint8_t* payload, uint32_t no
 
     if (!nack) {
       // ACKed SNs get marked and removed from tx_window so PDCP get's only notified once
+      std::lock_guard<std::mutex> lock(mutex);
       if (tx_window.has_sn(i)) {
         update_notification_ack_info(i);
         logger.debug("Tx PDU SN=%zd being removed from tx window", i);
@@ -1273,16 +1281,17 @@ void rlc_am_lte::rlc_am_lte_tx::handle_control_pdu(uint8_t* payload, uint32_t no
     i = (i + 1) % MOD;
   }
 
+  {
     // Make sure vt_a points to valid SN
+    std::lock_guard<std::mutex> lock(mutex);
     if (not tx_window.empty() && not tx_window.has_sn(vt_a)) {
       logger.error("%s vt_a=%d points to invalid position in Tx window.", RB_NAME, vt_a);
       parent->rrc->protocol_failure();
     }
+  }
 
   debug_state();
 
-  lock.unlock();
-
   // Notify PDCP without holding Tx mutex
   if (not notify_info_vec.empty()) {
     parent->pdcp->notify_delivery(parent->lcid, notify_info_vec);