The attached program can run either in a threaded mode or a multiprocess mode. It defaults to threaded mode, and switches to multiprocess mode if the first positional argument is "process". "success" of the test is defined as the final count being seen as 2000000 by both tasks.
In standard linux x86_64 userspace (i7, 4 cores) and in standard armhf userspace (4 cores), the test program consistently completes successfully in both modes. But with qemu arm-linux-user, the test consistently succeeds in threaded mode and generally fails in multiprocess mode.
The test reflects an essential aspect of how the Free and Open Source project linuxcnc's IPC system works: shared memory regions (created by shmat, but mmap would probably behave the same) contain data and mutexes. I observed that our testsuite encounters numerous deadlocks and failures when running in an schroot with qemu-user (x86_64 host), and I believe the underlying cause is improper support for atomic operations in a multiprocess model. (the testsuite consistently passes on real hardware)
I observed the same failure at v1.6.0 and master (v2.6.0-424-g287db79), as well as in the outdated Debian version 1:2.1+dfsg-12+deb8u5a.
Hi. Your test program doesn't work for me running natively (x86-64):
$ gcc -O -pthread -o /tmp/shmipc-native -static /tmp/shmipc.c
$ time /tmp/shmipc-native
threaded test
^C
real 929m16.382s
user 1858m14.140s
sys 0m2.924s
...I left it running overnight and it still hadn't finished when I got in in the morning, so I killed it.
Do you have a repro case that completes in a more reasonable timescale?