That is probably it. When there are 4 ranks there are 4 int64’s just before the 
user data (for PSCW). With 1 rank we don’t even bother, its just malloc 
(16-byte aligned). With any other odd number of ranks the user data is after an 
odd number of int64’s and is 8-byte aligned. There is no requirement in MPI to 
provide 16-byte alignment (which is required for _Atomic __int128 because of 
the alignment requirement of cmpxchg16b) so you have to align it yourself.

-Nathan

> On May 3, 2018, at 2:16 PM, Joseph Schuchart <schuch...@hlrs.de> wrote:
> 
> Martin,
> 
> You say that you allocate shared memory, do you mean shared memory windows? 
> If so, this could be the reason: https://github.com/open-mpi/ompi/issues/4952
> 
> The alignment of memory allocated for MPI windows is not suitable for 128bit 
> values in Open MPI (only 8-byte alignment is guaranteed atm). I have seen the 
> alignment change depending on the number of processes. Could you check the 
> alignment of the memory you are trying to access?
> 
> Cheers,
> Joseph
> 
> On 05/03/2018 08:48 PM, Martin Böhm wrote:
>> Dear all,
>> I have a problem with a segfault on a user-built OpenMPI 3.0.1 running on 
>> Ubuntu 16.04
>> in local mode (only processes on a single computer).
>> The problem manifests itself as a segfault when allocating shared memory for
>> (at least) one 128-bit atomic variable (say std::atomic<__int128>) and then
>> storing into this variable. This error only occurs when there are at least 
>> two
>> processes, even though only the process of shared rank 0 is the one doing 
>> both
>> the allocation and the writing. The compile flags -march=core2 or 
>> -march=native
>> or -latomic were tried, none of which helped.
>> An example of the code that triggers it on my computers is this:
>> https://github.com/bohm/binstretch/blob/parallel-classic/algorithm/bug.cpp
>> The code works fine with mpirun -np 1 and segfaults with mpirun -np 2, 3 and 
>> 4;
>> if line 41 is commented out (the 128-bit atomic write), everything works fine
>> with -np 2 or more.
>> As for Ubuntu's stock package containing OpenMPI 1.10.2, the code segfaults 
>> with
>> "-np 2" and "-np 3" but not "-np 1" or "-np 4".
>> Thank you for any assistance concerning this problem. I would suspect my own
>> code to be the most likely culprit, since it triggers on both the stock 
>> package
>> and custom-built OpenMPI.
>> I attach the config.log.bz2 and ompi_info.log. Below I list some runs of the 
>> program
>> and what errors are produced.
>> Thank you for any assistance. I have tried googling and searching the 
>> mailing list
>> for this problem; if I missed something, I apologize.
>> Martin Böhm
>> ----- Ubuntu 16.04 stock mpirun and mpic++ -----
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/bin/mpirun -np 1 ../tests/bug
>> ex1 success.
>> ex2 success.
>> Inserted into ex1.
>> Inserted into ex2.
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/bin/mpirun -np 4 ../tests/bug
>> Thread 2: ex1 success.
>> Thread 3: ex1 success.
>> ex1 success.
>> Thread 1: ex1 success.
>> ex2 success.
>> Thread 2: ex2 success.
>> Thread 1: ex2 success.
>> Thread 3: ex2 success.
>> Inserted into ex1.
>> Inserted into ex2.
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/bin/mpirun -np 2 ../tests/bug
>> Thread 1: ex1 success.
>> ex1 success.
>> Thread 1: ex2 success.
>> ex2 success.
>> Inserted into ex1.
>> [kamenice:13662] *** Process received signal ***
>> [kamenice:13662] Signal: Segmentation fault (11)
>> [kamenice:13662] Signal code:  (128)
>> [kamenice:13662] Failing at address: (nil)
>> [kamenice:13662] [ 0] 
>> /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7f31773844b0]
>> [kamenice:13662] [ 1] ../tests/bug[0x40d8ac]
>> [kamenice:13662] [ 2] ../tests/bug[0x408997]
>> [kamenice:13662] [ 3] ../tests/bug[0x408bf0]
>> [kamenice:13662] [ 4] 
>> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f317736f830]
>> [kamenice:13662] [ 5] ../tests/bug[0x4086e9]
>> [kamenice:13662] *** End of error message ***
>> ----- Ubuntu 16.04 custom-compiled OpenMPI 3.0.1, installed to /usr/local 
>> (the stock packages were uninstalled) -----
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 1 
>> ../tests/bug
>> ex1 success.
>> ex2 success.
>> Inserted into ex1.
>> Inserted into ex2.
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 2 
>> ../tests/bug
>> ex1 success.
>> Thread 1: ex1 success.
>> Inserted into ex1.
>> [kamenice:22794] *** Process received signal ***
>> ex2 success.
>> Thread 1: ex2 success.
>> [kamenice:22794] Signal: Segmentation fault (11)
>> [kamenice:22794] Signal code:  (128)
>> [kamenice:22794] Failing at address: (nil)
>> [kamenice:22794] [ 0] 
>> /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7ff8bad084b0]
>> [kamenice:22794] [ 1] ../tests/bug[0x401010]
>> [kamenice:22794] [ 2] ../tests/bug[0x400d27]
>> [kamenice:22794] [ 3] ../tests/bug[0x400f80]
>> [kamenice:22794] [ 4] 
>> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7ff8bacf3830]
>> [kamenice:22794] [ 5] ../tests/bug[0x400a79]
>> [kamenice:22794] *** End of error message ***
>> -------------------------------------------------------
>> Primary job  terminated normally, but 1 process returned
>> a non-zero exit code. Per user-direction, the job has been aborted.
>> -------------------------------------------------------
>> --------------------------------------------------------------------------
>> mpirun noticed that process rank 0 with PID 0 on node kamenice exited on 
>> signal 11 (Segmentation fault).
>> --------------------------------------------------------------------------
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 4 
>> --oversubscribe ../tests/bug
>> ex1 success.
>> Thread 1: ex1 success.
>> Thread 2: ex1 success.
>> Thread 3: ex1 success.
>> ex2 success.
>> Thread 1: ex2 success.
>> Thread 2: ex2 success.
>> Thread 3: ex2 success.
>> Inserted into ex1.
>> [kamenice:22728] *** Process received signal ***
>> [kamenice:22728] Signal: Segmentation fault (11)
>> [kamenice:22728] Signal code:  (128)
>> [kamenice:22728] Failing at address: (nil)
>> [kamenice:22728] [ 0] 
>> /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7f826a6294b0]
>> [kamenice:22728] [ 1] ../tests/bug[0x401010]
>> [kamenice:22728] [ 2] ../tests/bug[0x400d27]
>> [kamenice:22728] [ 3] ../tests/bug[0x400f80]
>> [kamenice:22728] [ 4] 
>> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f826a614830]
>> [kamenice:22728] [ 5] ../tests/bug[0x400a79]
>> [kamenice:22728] *** End of error message ***
>> -------------------------------------------------------
>> Primary job  terminated normally, but 1 process returned
>> a non-zero exit code. Per user-direction, the job has been aborted.
>> -------------------------------------------------------
>> --------------------------------------------------------------------------
>> mpirun noticed that process rank 0 with PID 0 on node kamenice exited on 
>> signal 11 (Segmentation fault).
>> --------------------------------------------------------------------------
>> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 2 
>> valgrind ../tests/bug
>> ==22814== Memcheck, a memory error detector
>> ==22814== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
>> ==22814== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
>> ==22814== Command: ../tests/bug
>> ==22814==
>> ==22815== Memcheck, a memory error detector
>> ==22815== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
>> ==22815== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
>> ==22815== Command: ../tests/bug
>> ==22815==
>> ex1 success.
>> Thread 1: ex1 success.
>> Thread 1: ex2 success.
>> ex2 success.
>> Inserted into ex1.
>> [kamenice:22814] *** Process received signal ***
>> [kamenice:22814] Signal: Segmentation fault (11)
>> [kamenice:22814] Signal code:  (128)
>> [kamenice:22814] Failing at address: (nil)
>> [kamenice:22814] [ 0] /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x51704b0]
>> [kamenice:22814] [ 1] ../tests/bug[0x401010]
>> [kamenice:22814] [ 2] ../tests/bug[0x400d27]
>> [kamenice:22814] [ 3] ../tests/bug[0x400f80]
>> [kamenice:22814] [ 4] 
>> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x515b830]
>> [kamenice:22814] [ 5] ../tests/bug[0x400a79]
>> [kamenice:22814] *** End of error message ***
>> ==22814==
>> ==22814== Process terminating with default action of signal 11 (SIGSEGV)
>> ==22814==    at 0x5170428: raise (raise.c:54)
>> ==22814==    by 0x5827E4D: show_stackframe (in 
>> /usr/local/lib/libopen-pal.so.40.1.0)
>> ==22814==    by 0x51704AF: ??? (in /lib/x86_64-linux-gnu/libc-2.23.so)
>> ==22814==    by 0x40100F: std::atomic<__int128>::store(__int128, 
>> std::memory_order) (atomic:225)
>> ==22814==    by 0x400D26: shared_memory_init(int, int) (bug.cpp:41)
>> ==22814==    by 0x400F7F: main (bug.cpp:80)
>> ==22814==
>> ==22814== HEAP SUMMARY:
>> ==22814==     in use at exit: 2,759,989 bytes in 9,014 blocks
>> ==22814==   total heap usage: 20,168 allocs, 11,154 frees, 3,820,707 bytes 
>> allocated
>> ==22814==
>> ==22814== LEAK SUMMARY:
>> ==22814==    definitely lost: 12 bytes in 1 blocks
>> ==22814==    indirectly lost: 0 bytes in 0 blocks
>> ==22814==      possibly lost: 608 bytes in 2 blocks
>> ==22814==    still reachable: 2,759,369 bytes in 9,011 blocks
>> ==22814==         suppressed: 0 bytes in 0 blocks
>> ==22814== Rerun with --leak-check=full to see details of leaked memory
>> ==22814==
>> ==22814== For counts of detected and suppressed errors, rerun with: -v
>> ==22814== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
>> -------------------------------------------------------
>> Primary job  terminated normally, but 1 process returned
>> a non-zero exit code. Per user-direction, the job has been aborted.
>> -------------------------------------------------------
>> ==22815==
>> ==22815== Process terminating with default action of signal 15 (SIGTERM)
>> ==22815==    at 0x523674D: ??? (syscall-template.S:84)
>> ==22815==    by 0x583B4A7: poll (poll2.h:46)
>> ==22815==    by 0x583B4A7: poll_dispatch (poll.c:165)
>> ==22815==    by 0x5831BDE: opal_libevent2022_event_base_loop (event.c:1630)
>> ==22815==    by 0x57F210D: progress_engine (in 
>> /usr/local/lib/libopen-pal.so.40.1.0)
>> ==22815==    by 0x5FD76B9: start_thread (pthread_create.c:333)
>> ==22815==    by 0x524241C: clone (clone.S:109)
>> ==22815==
>> ==22815== HEAP SUMMARY:
>> ==22815==     in use at exit: 2,766,405 bytes in 9,017 blocks
>> ==22815==   total heap usage: 20,167 allocs, 11,150 frees, 3,823,751 bytes 
>> allocated
>> ==22815==
>> ==22815== LEAK SUMMARY:
>> ==22815==    definitely lost: 12 bytes in 1 blocks
>> ==22815==    indirectly lost: 0 bytes in 0 blocks
>> ==22815==      possibly lost: 608 bytes in 2 blocks
>> ==22815==    still reachable: 2,765,785 bytes in 9,014 blocks
>> ==22815==         suppressed: 0 bytes in 0 blocks
>> ==22815== Rerun with --leak-check=full to see details of leaked memory
>> ==22815==
>> ==22815== For counts of detected and suppressed errors, rerun with: -v
>> ==22815== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
>> --------------------------------------------------------------------------
>> mpirun noticed that process rank 0 with PID 0 on node kamenice exited on 
>> signal 9 (Killed).
>> --------------------------------------------------------------------------
>> _______________________________________________
>> users mailing list
>> users@lists.open-mpi.org
>> https://lists.open-mpi.org/mailman/listinfo/users
> 
> 
> --
> Dipl.-Inf. Joseph Schuchart
> High Performance Computing Center Stuttgart (HLRS)
> Nobelstr. 19
> D-70569 Stuttgart
> 
> Tel.: +49(0)711-68565890
> Fax: +49(0)711-6856832
> E-Mail: schuch...@hlrs.de
> _______________________________________________
> users mailing list
> users@lists.open-mpi.org
> https://lists.open-mpi.org/mailman/listinfo/users

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