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

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Date:   Wed, 13 Nov 2019 10:04:14 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Harris, Robert" <robert.harris@...rtlogic.com>
cc:     Ingo Molnar <mingo@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dvhart@...radead.org" <dvhart@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns
 EPERM

On Tue, 12 Nov 2019, Harris, Robert 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.

sys_futex(FUTEX_WAIT_PRIVATE) does not return -EPERM. Only the PI variants
do that.

Thanks,

	tglx