diff --git a/opal/mca/btl/usnic/README.txt b/opal/mca/btl/usnic/README.txt
index 1af846b1c9..ab0b7d12b7 100644
--- a/opal/mca/btl/usnic/README.txt
+++ b/opal/mca/btl/usnic/README.txt
@@ -335,3 +335,40 @@ libfabric abstractions:
 fi_fabric: corresponds to a VIC PF
 fi_domain: corresponds to a VIC VF
 fi_endpoint: resources inside the VIC VF (basically a QP)
+
+======================================
+
+MPI_THREAD_MULTIPLE support
+
+In order to make usnic btl thread-safe, the mutex locks are issued
+to protect the critical path. ie; libfabric routines, book keeping, etc.
+
+The said lock is btl_usnic_lock. It is a RECURSIVE lock, meaning that
+the same thread can take the lock again even if it already has the lock to
+allow the callback function to post another segment right away if we know
+that the current segment is completed inline. (So we can call send in send
+without deadlocking)
+
+These two functions taking care of hotel checkin/checkout and we
+have to protect that part. So we take the mutex lock before we enter the
+function.
+
+- opal_btl_usnic_check_rts()
+- opal_btl_usnic_handle_ack()
+
+We also have to protect the call to libfabric routines
+
+- opal_btl_usnic_endpoint_send_segment()        (fi_send)
+- opal_btl_usnic_recv_call()			(fi_recvmsg)
+
+have to be protected as well.
+
+Also cclient connection checking (opal_btl_usnic_connectivity_ping) has to be
+protected. This happens only in the beginning but cclient communicate with cagent
+through opal_fd_read/write() and if two or more clients do opal_fd_write() at the
+same time, the data might be corrupt.
+
+With this concept, many functions in btl/usnic that make calls to the
+listed functions are protected by OPAL_THREAD_LOCK macro which will only
+be active if the user specify MPI_Init_thread() with MPI_THREAD_MULTIPLE
+support.
diff --git a/opal/mca/btl/usnic/btl_usnic_cclient.c b/opal/mca/btl/usnic/btl_usnic_cclient.c
index d76b3b8ca9..6370c2c790 100644
--- a/opal/mca/btl/usnic/btl_usnic_cclient.c
+++ b/opal/mca/btl/usnic/btl_usnic_cclient.c
@@ -228,7 +228,8 @@ int opal_btl_usnic_connectivity_ping(uint32_t src_ipv4_addr, int src_port,
                                      uint32_t dest_netmask, int dest_port,
                                      char *dest_nodename,
                                      size_t max_msg_size)
-{
+{  
+    OPAL_THREAD_LOCK(&btl_usnic_lock);
     /* If connectivity checking is not enabled, do nothing */
     if (!mca_btl_usnic_component.connectivity_enabled) {
         return OPAL_SUCCESS;
@@ -259,6 +260,7 @@ int opal_btl_usnic_connectivity_ping(uint32_t src_ipv4_addr, int src_port,
         ABORT("usnic connectivity client IPC write failed");
         /* Will not return */
     }
+    OPAL_THREAD_UNLOCK(&btl_usnic_lock);
 
     return OPAL_SUCCESS;
 }
diff --git a/opal/mca/btl/usnic/btl_usnic_compat.c b/opal/mca/btl/usnic/btl_usnic_compat.c
index 1289093ac1..2346dc56c5 100644
--- a/opal/mca/btl/usnic/btl_usnic_compat.c
+++ b/opal/mca/btl/usnic/btl_usnic_compat.c
@@ -509,6 +509,7 @@ opal_btl_usnic_prepare_src(
     size_t* size,
     uint32_t flags)
 {
+    OPAL_THREAD_LOCK(&btl_usnic_lock);
     opal_btl_usnic_module_t *module = (opal_btl_usnic_module_t*) base_module;
     opal_btl_usnic_send_frag_t *frag;
     uint32_t payload_len;
@@ -552,6 +553,7 @@ opal_btl_usnic_prepare_src(
 #endif
 #endif
 
+    OPAL_THREAD_UNLOCK(&btl_usnic_lock);
     return &frag->sf_base.uf_base;
 }
 
diff --git a/opal/mca/btl/usnic/btl_usnic_component.c b/opal/mca/btl/usnic/btl_usnic_component.c
index 07803ce9ee..8a3b6e1800 100644
--- a/opal/mca/btl/usnic/btl_usnic_component.c
+++ b/opal/mca/btl/usnic/btl_usnic_component.c
@@ -86,6 +86,9 @@
 
 #define OPAL_BTL_USNIC_NUM_COMPLETIONS 500
 
+/* MPI_THREAD_MULTIPLE_SUPPORT */
+opal_recursive_mutex_t btl_usnic_lock;
+
 /* RNG buffer definition */
 opal_rng_buff_t opal_btl_usnic_rand_buff = {0};
 
@@ -222,6 +225,8 @@ static int usnic_component_close(void)
     opal_btl_usnic_cleanup_tests();
 #endif
 
+    OBJ_DESTRUCT(&btl_usnic_lock);
+
     return OPAL_SUCCESS;
 }
 
@@ -615,13 +620,22 @@ static mca_btl_base_module_t** usnic_component_init(int* num_btl_modules,
 
     *num_btl_modules = 0;
 
-    /* Currently refuse to run if MPI_THREAD_MULTIPLE is enabled */
+    /* MPI_THREAD_MULTIPLE is only supported in 2.0+ */
     if (want_mpi_threads && !mca_btl_base_thread_multiple_override) {
-        opal_output_verbose(5, USNIC_OUT,
-                            "btl:usnic: MPI_THREAD_MULTIPLE not supported; skipping this component");
-        return NULL;
+
+	if (OMPI_MAJOR_VERSION >= 2) {
+            opal_output_verbose(5, USNIC_OUT,
+                                "btl:usnic: MPI_THREAD_MULTIPLE support is in testing phase.");
+	}
+	else {
+            opal_output_verbose(5, USNIC_OUT,
+                                "btl:usnic: MPI_THREAD_MULTIPLE is not supported in version < 2.");
+	    return NULL;
+	}
     }
 
+    OBJ_CONSTRUCT(&btl_usnic_lock, opal_recursive_mutex_t);
+
     /* We only want providers named "usnic that are of type EP_DGRAM */
     fabric_attr.prov_name = "usnic";
     ep_attr.type = FI_EP_DGRAM;
@@ -1151,6 +1165,8 @@ static int usnic_handle_completion(
     /* Make the completion be Valgrind-defined */
     opal_memchecker_base_mem_defined(seg, sizeof(*seg));
 
+    OPAL_THREAD_LOCK(&btl_usnic_lock);
+
     /* Handle work completions */
     switch(seg->us_type) {
 
@@ -1181,6 +1197,8 @@ static int usnic_handle_completion(
         BTL_ERROR(("Unhandled completion segment type %d", seg->us_type));
         break;
     }
+
+    OPAL_THREAD_UNLOCK(&btl_usnic_lock);
     return 1;
 }
 
diff --git a/opal/mca/btl/usnic/btl_usnic_module.c b/opal/mca/btl/usnic/btl_usnic_module.c
index 786f444da4..7cca5354ab 100644
--- a/opal/mca/btl/usnic/btl_usnic_module.c
+++ b/opal/mca/btl/usnic/btl_usnic_module.c
@@ -1086,6 +1086,7 @@ opal_btl_usnic_module_progress_sends(
     /*
      * Handle all the retransmits we can
      */
+    OPAL_THREAD_LOCK(&btl_usnic_lock);
     if (OPAL_UNLIKELY(!opal_list_is_empty(&module->pending_resend_segs))) {
         usnic_do_resends(module);
     }
@@ -1195,6 +1196,7 @@ opal_btl_usnic_module_progress_sends(
 
         endpoint = next_endpoint;
     }
+    OPAL_THREAD_UNLOCK(&btl_usnic_lock);
 }
 
 /*
@@ -1229,6 +1231,7 @@ usnic_send(
     opal_btl_usnic_module_t *module;
     opal_btl_usnic_send_segment_t *sseg;
 
+    OPAL_THREAD_LOCK(&btl_usnic_lock);
     endpoint = (opal_btl_usnic_endpoint_t *)base_endpoint;
     module = (opal_btl_usnic_module_t *)base_module;
     frag = (opal_btl_usnic_send_frag_t*) descriptor;
@@ -1337,6 +1340,7 @@ usnic_send(
 
     ++module->stats.pml_module_sends;
 
+    OPAL_THREAD_UNLOCK(&btl_usnic_lock);
     return rc;
 }
 
diff --git a/opal/mca/btl/usnic/btl_usnic_module.h b/opal/mca/btl/usnic/btl_usnic_module.h
index b7f49c596b..15cc1933ef 100644
--- a/opal/mca/btl/usnic/btl_usnic_module.h
+++ b/opal/mca/btl/usnic/btl_usnic_module.h
@@ -53,6 +53,12 @@
 
 BEGIN_C_DECLS
 
+/*
+ * MPI_THREAD_MULTIPLE support
+ */
+extern opal_recursive_mutex_t btl_usnic_lock;
+
+
 /*
  * Forward declarations to avoid include loops
  */
diff --git a/opal/mca/btl/usnic/btl_usnic_recv.c b/opal/mca/btl/usnic/btl_usnic_recv.c
index c77388ef23..443e2b0e96 100644
--- a/opal/mca/btl/usnic/btl_usnic_recv.c
+++ b/opal/mca/btl/usnic/btl_usnic_recv.c
@@ -340,8 +340,9 @@ void opal_btl_usnic_recv_call(opal_btl_usnic_module_t *module,
         opal_output(0, "    Received ACK for sequence number %" UDSEQ " from %s to %s\n",
                     bseg->us_btl_header->ack_seq, remote_ip, local_ip);
 #endif
+        OPAL_THREAD_LOCK(&btl_usnic_lock);
         opal_btl_usnic_handle_ack(endpoint, ack_seq);
-
+        OPAL_THREAD_UNLOCK(&btl_usnic_lock);
         goto repost;
     }
 
diff --git a/opal/mca/btl/usnic/btl_usnic_recv.h b/opal/mca/btl/usnic/btl_usnic_recv.h
index 4773bba4aa..70ffa7d4db 100644
--- a/opal/mca/btl/usnic/btl_usnic_recv.h
+++ b/opal/mca/btl/usnic/btl_usnic_recv.h
@@ -157,8 +157,10 @@ opal_btl_usnic_check_rx_seq(
 #if MSGDEBUG1
         opal_output(0, "Handle piggy-packed ACK seq %"UDSEQ"\n", seg->rs_base.us_btl_header->ack_seq);
 #endif
+        OPAL_THREAD_LOCK(&btl_usnic_lock);
         opal_btl_usnic_handle_ack(endpoint,
                 seg->rs_base.us_btl_header->ack_seq);
+        OPAL_THREAD_UNLOCK(&btl_usnic_lock);
     }
 
     /* Do we have room in the endpoint's receiver window?