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

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Message-ID: <alpine.DEB.2.21.1911132220090.2507@nanos.tec.linutronix.de>
Date:   Wed, 13 Nov 2019 22:34:50 +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 Wed, 13 Nov 2019, Harris, Robert wrote:
> > On 13 Nov 2019, at 09:04, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Tue, 12 Nov 2019, Harris, Robert wrote:
> >> 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.
> 
> In that case I would appreciate a second pair of eyes.  The error I see
> (intermittently) is

The code looks innocent enough. As I don't know whether the kernel version
you mentioned is a vanilla 4.19.184 from the stable tree or some patched up
frankenkernel which pretends to have this version number, I can't be sure
that this is an issue in that particular kernel.

In the vanilla 4.19.184 I really cant find how that would return EPERM for
regular futexes.

Thanks,

	tglx