linux-kernel - Re: BUG_ON(!newowner) in fixup_pi_state

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Message-ID: <871rh9mkvr.fsf@nanos.tec.linutronix.de>
Date:   Wed, 04 Nov 2020 12:17:28 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mike Galbraith <efault@....de>,
        Gratian Crisan <gratian.crisan@...com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Brandon Streiff <brandon.streiff@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        James Minor <james.minor@...com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: BUG_ON(!newowner) in fixup_pi_state_owner()

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....