add mutex in pdcp entity

write_sdu and write_pdu are not re-entrant because all functions
that deal with the counters, e.g. integrity check and generate,
must be protected because MAC threads may call them at the same time.

this addresses issue #254

lib/include/srslte/upper/pdcp_entity.h

@@ -64,6 +64,7 @@ class pdcp_entity : public pdcp_entity_interface
 {
 public:
   pdcp_entity();
+  ~pdcp_entity();
   void init(srsue::rlc_interface_pdcp     *rlc_,
             srsue::rrc_interface_pdcp     *rrc_,
             srsue::gw_interface_pdcp      *gw_,
@@ -112,6 +113,8 @@ private:
   CIPHERING_ALGORITHM_ID_ENUM cipher_algo;
   INTEGRITY_ALGORITHM_ID_ENUM integ_algo;
 
+  pthread_mutex_t mutex;
+
   void integrity_generate(uint8_t  *msg,
                           uint32_t  msg_len,
                           uint8_t  *mac);

lib/src/upper/pdcp_entity.cc

@@ -46,6 +46,12 @@ pdcp_entity::pdcp_entity()
   rx_count = 0;
   cipher_algo = CIPHERING_ALGORITHM_ID_EEA0;
   integ_algo = INTEGRITY_ALGORITHM_ID_EIA0;
+  pthread_mutex_init(&mutex, NULL);
+}
+
+pdcp_entity::~pdcp_entity()
+{
+  pthread_mutex_destroy(&mutex);
 }
 
 void pdcp_entity::init(srsue::rlc_interface_pdcp      *rlc_,
@@ -119,6 +125,8 @@ void pdcp_entity::write_sdu(byte_buffer_t *sdu, bool blocking)
         rrc->get_rb_name(lcid).c_str(), tx_count,
         (do_integrity) ? "true" : "false", (do_encryption) ? "true" : "false");
 
+  pthread_mutex_lock(&mutex);
+
   if (cfg.is_control) {
     pdcp_pack_control_pdu(tx_count, sdu);
     if(do_integrity) {
@@ -144,6 +152,8 @@ void pdcp_entity::write_sdu(byte_buffer_t *sdu, bool blocking)
   }
   tx_count++;
 
+  pthread_mutex_unlock(&mutex);
+
   rlc->write_sdu(lcid, sdu, blocking);
 }
 
@@ -183,6 +193,8 @@ void pdcp_entity::write_pdu(byte_buffer_t *pdu)
     return;
   }
 
+  pthread_mutex_lock(&mutex);
+
   // Handle DRB messages
   if (cfg.is_data) {
     uint32_t sn;
@@ -231,6 +243,7 @@ void pdcp_entity::write_pdu(byte_buffer_t *pdu)
   }
 exit:
   rx_count++;
+  pthread_mutex_unlock(&mutex);
 }
 
 void pdcp_entity::integrity_generate( uint8_t  *msg,