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From:   "Harris, Robert" <robert.harris@alertlogic.com>
To:     "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "dvhart@infradead.org" <dvhart@infradead.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Harris, Robert" <robert.harris@alertlogic.com>
Subject: Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns EPERM
Thread-Topic: Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns
 EPERM
Thread-Index: AQHVmYBWNvZci+jLJUqod1Huijtj9w==
Date:   Tue, 12 Nov 2019 17:40:55 +0000
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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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