** Changed in: qemu
Status: Expired => Fix Committed
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https://bugs.launchpad.net/bugs/1585840
Title:
multiprocess program gets incorrect results with qemu arm-linux-user
Status in QEMU:
Fix Committed
Bug description:
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(v2.6.0-424-g287db79), as well as in the outdated Debian version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.
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