linux-kernel - Re: Several tst-robust* tests time out with recent Linux kernel

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Message-ID: <564119521b61b5a38f9bdfe6c7a41fcbb07049c9.camel@intel.com>
Date:   Wed, 15 Nov 2023 23:28:00 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "xry111@...111.site" <xry111@...111.site>,
        "andrealmeid@...lia.com" <andrealmeid@...lia.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Several tst-robust* tests time out with recent Linux kernel

On Wed, 2023-11-15 at 09:51 +0100, Peter Zijlstra wrote:
> On Wed, Nov 15, 2023 at 01:11:20AM +0000, Edgecombe, Rick P wrote:
> > On Tue, 2023-11-14 at 21:14 +0100, Peter Zijlstra wrote:
> > > Urgh, thanks!
> > > 
> > > Confirmed, the below cures things. Although I should probably
> > > make
> > > that
> > > FLAGS_SIZE_32 | FLAGS_SHARED against Linus' tree.
> > > 
> > > Let me go do a proper patch.
> > 
> > I saw these fail on the glibc shadow stack branch today, and I also
> > saw
> > this one failing:
> > FAIL: nptl/tst-robustpi8
> 
> tip/locking/urgent (branch with the fix on) gets me:
> 
> root@...-ep:/usr/local/src/glibc# ./build/nptl/tst-robustpi8 
> running child
> verifying locks
> running child
> verifying locks
> running child
> verifying locks
> running child
> verifying locks
> running child
> verifying locks
> root@...-ep:/usr/local/src/glibc#
> 
> Which, to my untrained eye, looks like a pass to me.

It bisects to this for me:
fbeb558b0dd0 ("futex/pi: Fix recursive rt_mutex waiter state")

Reading the patch, I'm not immediately clear what is going on but a few
comments stood out: "There be dragons here" "What could possibly go
wrong..." "This is a somewhat dangerous proposition".

Seems a likelihood of some race, but it reproduces reliably on my
machine. Haven't dug into debugging it yet. Any pointers?