Re: [PATCH V2] lockdep: fix deadlock issue between lockdep and rcu

[Bart Van Assche <bvanassche@acm.org>]

On 2/1/24 11:58, Boqun Feng wrote:
> On Thu, Feb 01, 2024 at 09:22:20AM -0800, Bart Van Assche wrote:
>> On 1/16/24 23:48, Zhiguo Niu wrote:
>>>    	/*
>>> -	 * If there's anything on the open list, close and start a new callback.
>>> -	 */
>>> -	call_rcu_zapped(delayed_free.pf + delayed_free.index);
>>> +	* If there's anything on the open list, close and start a new callback.
>>> +	*/
>>> +	if (need_callback)
>>> +		call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>>
>> The comment above the if-statement refers to the call_rcu_zapped() function
>> while call_rcu_zapped() has been changed into call_rcu(). So the comment is
>> now incorrect.
>>
>> Additionally, what guarantees that the above code won't be triggered
>> concurrently from two different threads? As you may know calling call_rcu()
>> twice before the callback has been started is not allowed. I think that can
>> happen with the above code.
> 
> No, it's synchronized by the delayed_free.schedule. Only one thread/CPU
> can schedule at a time. Or am I missing something subtle?

Only call_rcu_zapped() reads and modifies delayed_free.scheduled. Direct
call_rcu() calls do neither read nor modify delayed_free.scheduled.

Bart.