Re: [PATCH] rtmutex: ensure we wake up the top waiter

[Waiman Long <longman@redhat.com>]

On 1/17/23 14:32, Waiman Long wrote:
> On 1/17/23 12:26, Wander Lairson Costa wrote:
>> In task_blocked_on_lock() we save the owner, release the wait_lock and
>> call rt_mutex_adjust_prio_chain(). Before we acquire the wait_lock
>> again, the owner may release the lock and deboost.
> Are you referring to task_blocks_on_rt_mutex(), not 
> task_blocked_on_lock()?
>>
>> rt_mutex_adjust_prio_chain() acquires the wait_lock. In the requeue
>> phase, waiter may be initially in the top of the queue, but after
>> dequeued and requeued it may no longer be true.
>>
>> This scenario ends up waking the wrong task, which will verify it is no
>> the top waiter and comes back to sleep. Now we have a situation in which
>> no task is holding the lock but no one acquires it.
>>
>> We can reproduce the bug in PREEMPT_RT with stress-ng:
>>
>> while true; do
>>      stress-ng --sched deadline --sched-period 1000000000 \
>>              --sched-runtime 800000000 --sched-deadline \
>>              1000000000 --mmapfork 23 -t 20
>> done
>>
>> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
>> ---
>>   kernel/locking/rtmutex.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>> index 010cf4e6d0b8..728f434de2bb 100644
>> --- a/kernel/locking/rtmutex.c
>> +++ b/kernel/locking/rtmutex.c
>> @@ -901,8 +901,9 @@ static int __sched 
>> rt_mutex_adjust_prio_chain(struct task_struct *task,
>>            * then we need to wake the new top waiter up to try
>>            * to get the lock.
>>            */
>> -        if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
>> -            wake_up_state(waiter->task, waiter->wake_state);
>> +        top_waiter = rt_mutex_top_waiter(lock);
>> +        if (prerequeue_top_waiter != top_waiter)
>> +            wake_up_state(top_waiter->task, top_waiter->wake_state);
>>           raw_spin_unlock_irq(&lock->wait_lock);
>>           return 0;
>>       }
>
> I would say that if a rt_mutex has no owner but have waiters, we 
> should always wake up the top waiter whether or not it is the current 
> waiter. So what is the result of the stress-ng run above? Is it a 
> hang? It is not clear from your patch description.

BTW, if it is a hang. What arch has this problem? x86 or arm64? There is 
a recent report of some rt_mutex locking issue in arm64, I believe. I 
don't know if it will be related. So it is important to know in what 
arch you see this problem.

Cheers,
Longman