Update memory barriers in shared memory (sm) BTL. Fixes periodic segmentation faults when using Intel 10.0.025 or PGI 9.0-3. See #2619 for more details. Thanks Brian and George!

This commit was SVN r24095.

ompi/mca/btl/sm/btl_sm.h

@@ -334,12 +334,13 @@ static inline int sm_fifo_write(void *value, sm_fifo_t *fifo)
     volatile void **q = (volatile void **) RELATIVE2VIRTUAL(fifo->queue);
 
     /* if there is no free slot to write, report exhausted resource */
+    opal_atomic_rmb();
     if ( SM_FIFO_FREE != q[fifo->head] )
         return OMPI_ERR_OUT_OF_RESOURCE;
 
     /* otherwise, write to the slot and advance the head index */
-    opal_atomic_rmb();
     q[fifo->head] = value;
+    opal_atomic_wmb();
     fifo->head = (fifo->head + 1) & fifo->mask;
     opal_atomic_wmb(); 
     return OMPI_SUCCESS;