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

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Message-ID: <c485e96c-6aed-4c4c-bbba-de5c8784be69@acm.org>
Date: Thu, 1 Feb 2024 12:56:36 -0800
From: Bart Van Assche <bvanassche@....org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Zhiguo Niu <zhiguo.niu@...soc.com>, peterz@...radead.org,
 mingo@...hat.com, will@...nel.org, longman@...hat.com,
 linux-kernel@...r.kernel.org, niuzhiguo84@...il.com, ke.wang@...soc.com,
 xuewen.yan@...soc.com, Carlos Llamas <cmllamas@...gle.com>
Subject: Re: [PATCH V2] lockdep: fix deadlock issue between lockdep and rcu

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.