From: Thomas Gleixner <tglx@linutronix.de>
To: Mike Galbraith <efault@gmx.de>,
Gratian Crisan <gratian.crisan@ni.com>,
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>
Subject: Re: BUG_ON(!newowner) in fixup_pi_state_owner()
Date: Wed, 04 Nov 2020 14:26:16 +0100 [thread overview]
Message-ID: <87v9ell0cn.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <871rh9mkvr.fsf@nanos.tec.linutronix.de>
On Wed, Nov 04 2020 at 12:17, Thomas Gleixner wrote:
> On Wed, Nov 04 2020 at 11:24, Thomas Gleixner wrote:
>> On Wed, Nov 04 2020 at 08:42, Mike Galbraith wrote:
>>> On Wed, 2020-11-04 at 01:56 +0100, Mike Galbraith wrote:
>>> --- 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 = 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 = -EAGAIN;
>>> + goto handle_err;
>>
>> Yes, that cures it, but does not really explain why newowner is
>> NULL. Lemme stare more.
>
> Aside of that it's going to create inconsistent state in the worst
> case. There is something really fishy in the trace Gratian provided....
Ok. With some more info added to the trace I could convince myself, that
Gratians trace is not so fishy at all and that Mike's fix is actually
the right thing to do.
I'll post that with a proper comment and changelog. Mike, can I add your
Signed-off-by to the thing?
next prev parent reply other threads:[~2020-11-04 13:26 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-28 23:24 BUG_ON(!newowner) in fixup_pi_state_owner() Gratian Crisan
2020-11-03 23:31 ` Gratian Crisan
2020-11-04 0:56 ` Mike Galbraith
2020-11-04 7:42 ` Mike Galbraith
2020-11-04 10:24 ` Thomas Gleixner
2020-11-04 11:17 ` Thomas Gleixner
2020-11-04 13:26 ` Thomas Gleixner [this message]
2020-11-04 13:43 ` Mike Galbraith
2020-11-04 15:12 ` [PATCH] futex: Handle transient "ownerless" rtmutex state correctly Thomas Gleixner
2020-11-04 15:22 ` Thomas Gleixner
2020-11-04 16:50 ` Mike Galbraith
2020-11-05 6:32 ` Gratian Crisan
2020-11-04 10:00 ` BUG_ON(!newowner) in fixup_pi_state_owner() Thomas Gleixner
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