pml/ob1: fix deadlock with communicator flag ALLOW_OVERTAKE.

We missed an assert to check if ALLOW_OVERTAKE is set or not before
validating the sequence number and this will cause deadlock.

Signed-off-by: Thananon Patinyasakdikul <tpatinya@utk.edu>

ompi/mca/pml/ob1/pml_ob1_recvfrag.c

@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2018 The University of Tennessee and The University
+ * Copyright (c) 2004-2019 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2007 High Performance Computing Center Stuttgart,
@@ -963,6 +963,7 @@ static int mca_pml_ob1_recv_frag_match( mca_btl_base_module_t *btl,
     frag_msg_seq = hdr->hdr_seq;
     next_msg_seq_expected = (uint16_t)proc->expected_sequence;
 
+    if (!OMPI_COMM_CHECK_ASSERT_ALLOW_OVERTAKE(comm_ptr)) {
         /* If the sequence number is wrong, queue it up for later. */
         if(OPAL_UNLIKELY(frag_msg_seq != next_msg_seq_expected)) {
             mca_pml_ob1_recv_frag_t* frag;
@@ -977,6 +978,7 @@ static int mca_pml_ob1_recv_frag_match( mca_btl_base_module_t *btl,
             OB1_MATCHING_UNLOCK(&comm->matching_lock);
             return OMPI_SUCCESS;
         }
+    }
 
     /* mca_pml_ob1_recv_frag_match_proc() will release the lock. */
     return mca_pml_ob1_recv_frag_match_proc(btl, comm_ptr, proc, hdr,
@@ -1011,6 +1013,10 @@ mca_pml_ob1_recv_frag_match_proc( mca_btl_base_module_t *btl,
 
  match_this_frag:
     /* We're now expecting the next sequence number. */
+    /* NOTE: We should have checked for ALLOW_OVERTAKE comm flag here
+     * but adding a branch in this critical path is not ideal for performance.
+     * We decided to let it run the sequence number even we are not doing
+     * anything with it. */
     proc->expected_sequence++;
 
     /* We generate the SEARCH_POSTED_QUEUE only when the message is