Thread 2 (Thread 0x42476940 (LWP 1023)):
#0  0x0000003cc260cd01 in sem_wait () from /lib64/libpthread.so.0
#1  0x00000000004633b7 in ckpt_cb ()
#2  0x0000003cc2a03b11 in cr_checkpoint () from /usr/lib64/libcr.so.0
#3  0x0000003cc2a03356 in ?? () from /usr/lib64/libcr.so.0
#4  0x0000003cc260673d in start_thread () from /lib64/libpthread.so.0
#5  0x0000003cc1ed3d1d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x2ad83ff0af40 (LWP 1021)):
#0  0x0000003cc1ebaa27 in sched_yield () from /lib64/libc.so.6
#1  0x000000000043ac4e in MPIU_Thread_CS_yield_lockname_recursive_impl_ ()
#2  0x000000000043debb in MPIDI_CH3I_Progress ()
#3  0x00000000004489cd in MPIDI_Win_fence ()
#4  0x000000000042d5da in PMPI_Win_fence ()
#5  0x0000000000408ced in IMB_accumulate ()
#6  0x0000000000402938 in main ()