Re: [PATCH 2/2] rcu/nocb: Fix missed RCU barrier on deoffloading

[Neeraj Upadhyay]

On 11/13/2024 3:07 AM, Frederic Weisbecker wrote:
> Le Mon, Nov 11, 2024 at 01:07:16PM +0530, Neeraj Upadhyay a écrit :
>>
>>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
>>> index 16865475120b..2605dd234a13 100644
>>> --- a/kernel/rcu/tree_nocb.h
>>> +++ b/kernel/rcu/tree_nocb.h
>>> @@ -891,7 +891,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>>>  	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
>>>  					    nocb_cb_wait_cond(rdp));
>>>  	if (kthread_should_park()) {
>>> -		kthread_parkme();
>>> +		/*
>>> +		 * kthread_park() must be preceded by an rcu_barrier().
>>> +		 * But yet another rcu_barrier() might have sneaked in between
>>> +		 * the barrier callback execution and the callbacks counter
>>> +		 * decrement.
>>> +		 */
>>> +		if (rdp->nocb_cb_sleep) {
>>
>> Is READ_ONCE() not required here?
> 
> No because it can't be written concurrently at this point. The value observed
> here if kthread_should_park() must have been written locally on the previous
> call to nocb_cb_wait().
> 

Ok, got it. I was not aware of any other flow (other than the one described in
this fix) which can race with it. So, asked.



- Neeraj