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

[Harris, Robert]

> On 12 Nov 2019, at 17:40, Harris, Robert <robert.harris@xxxxxxxxxxxxxx> wrote:
>
> 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.

To be clear, I do mean that futex() is returning -1 and setting errno
to EPERM.

Robert
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