Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns EPERM

[Harris, Robert]

I am investigating an issue on 4.9.184 in which futex() returns EPERM
intermittently for

futex(uaddr, FUTEX_WAIT_PRIVATE, val, &timeout, NULL, 0)

The failure affects an application in an AWS lambda;  traditional
debugging approaches vary from difficult to impossible.  I cannot
reproduce the problem at will, instrument the kernel, install a new
kernel or get an application core dump.

Understanding the circumstances under which EPERM can be returned for
FUTEX_WAIT_PRIVATE would be useful but it is not a documented failure
mode.  I have spent some time looking through futex.c but have not
found anything yet.  I would be grateful for a hint from someone more
knowledgeable.

Please address/cc me on any reply.

Thanks,

Robert Harris
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