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

[Harris, Robert]

> On 13 Nov 2019, at 13:29, Mikael Pettersson <mikpelinux@xxxxxxxxx> wrote:
>
> On Tue, Nov 12, 2019 at 6:43 PM 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.
>
>
> I just wanted to add that a colleague of mine reported the exact same
> issue to me two days ago: a highly threaded application (the Erlang
> VM) running in AWS lambda, futex wait calls occasionally failing with
> EPERM.  I don't have more specifics than that, I've asked for kernel
> version and the exact parameters in the failed futex call.

Thanks, that's a great data point.  One of my outstanding questions had
been "why does this happen to only us?"

When I look at the timings I can say with some confidence that the
problem stopped for us minutes after

2017 on 2019-10-23 in us-east-1
2030 on 2019-10-24 in eu-west-1
1817 on 2019-10-25 in us-west-2

(all times UTC).  I've logged a ticket with Amazon to find out what
changed.

Robert
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