linux-kernel - Re: Help requested: futex(..., FUTEX_WAIT

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Message-ID: <0FFCBE3B-65D0-493F-9DB6-D4D64218EB0D@alertlogic.com>
Date:   Wed, 13 Nov 2019 07:29:33 +0000
From:   "Harris, Robert" <robert.harris@...rtlogic.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dvhart@...radead.org" <dvhart@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns EPERM

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