A bit more info... The error returned to userspace is originating from: https://github.com/torvalds/linux/blob/master/kernel/futex/pi.c#L295 'uval' is often zero in that error case, but sometimes just a mismatching value like: uval=0x567, task_pid_vnr()=0x564 Depending on the number of CPUs the VM is running on it reproduces or not. When it does reproduce, the newly added path here is taken: https://github.com/torvalds/linux/blob/master/kernel/futex/pi.c#L1185 The path is taken a lot during the test, sometimes >400 times before the above linked error is generated during the syscall. When it doesn't reproduce, I never saw that new path taken. More print statements make the reproduction less reliable, so it does seem to have a race in the mix at least somewhat. Otherwise, I haven't tried to understand what is going on here with all this highwire locking. Hope it helps.
