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

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Message-ID: <E0332978-739B-4546-9C3F-975216C349D2@alertlogic.com>
Date:   Tue, 12 Nov 2019 17:40:55 +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>,
        "Harris, Robert" <robert.harris@...rtlogic.com>
Subject: Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns EPERM

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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