Re: BUG_ON(!newowner) in fixup_pi_state_owner()

[Thomas Gleixner]

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....