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Message-ID: <5f536491708682fc3b86cb5b7bc1e05ffa3521e7.camel@gmx.de>
Subject: Re: BUG_ON(!newowner) in fixup_pi_state_owner()
From:   Mike Galbraith <efault@gmx.de>
To:     Gratian Crisan <gratian.crisan@ni.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Brandon Streiff <brandon.streiff@ni.com>,
        Ingo Molnar <mingo@redhat.com>,
        Darren Hart <dvhart@infradead.org>,
        James Minor <james.minor@ni.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Wed, 04 Nov 2020 08:42:36 +0100
In-Reply-To: <2376f4e71c638aee215a4911e5efed14c5adf56e.camel@gmx.de>
References: <87a6w6x7bb.fsf@ni.com> <878sbixbk4.fsf@ni.com>
         <2376f4e71c638aee215a4911e5efed14c5adf56e.camel@gmx.de>
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On Wed, 2020-11-04 at 01:56 +0100, Mike Galbraith wrote:
> On Tue, 2020-11-03 at 17:31 -0600, Gratian Crisan wrote:
> > Hi all,
> >
> > I apologize for waking up the futex demons (and replying to my own
> > email), but ...
> >
> > Gratian Crisan writes:
> > >
> > > Brandon and I have been debugging a nasty race that leads to
> > > BUG_ON(!newowner) in fixup_pi_state_owner() in futex.c. So far
> > > we've only been able to reproduce the issue on 4.9.y-rt kernels.
> > > We are still testing if this is a problem for later RT branches.
> >
> > I was able to reproduce the BUG_ON(!newowner) in fixup_pi_state_owner(=
)
> > with a 5.10.0-rc1-rt1 kernel (currently testing 5.10.0-rc2-rt4).
>
> My box says it's generic.

=2D--
 kernel/futex.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

=2D-- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2383,7 +2383,18 @@ static int fixup_pi_state_owner(u32 __us
 		 * Since we just failed the trylock; there must be an owner.
 		 */
 		newowner =3D rt_mutex_owner(&pi_state->pi_mutex);
-		BUG_ON(!newowner);
+
+		/*
+		 * Why? Because I know what I'm doing with these beasts?  Nope,
+		 * but what the hell, a busy restart loop let f_boosted become
+		 * owner, so go for it. Box still boots, works, no longer makes
+		 * boom with fbomb_v2, and as an added bonus, didn't even blow
+		 * futextests all up.  Maybe it'll help... or not, we'll see.
+		 */
+		if (unlikely(!newowner)) {
+			err =3D -EAGAIN;
+			goto handle_err;
+		}
 	} else {
 		WARN_ON_ONCE(argowner !=3D current);
 		if (oldowner =3D=3D current) {