linux-kernel - Re: [PATCH] locking/lockdep: Avoid potential access of invalid memory in lock

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Message-ID: <4d3b818e-2aca-a596-644f-28e9e52c27bd@redhat.com>
Date:   Mon, 3 Jan 2022 13:17:01 -0500
From:   Waiman Long <longman@...hat.com>
To:     Bart Van Assche <bvanassche@....org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH] locking/lockdep: Avoid potential access of invalid memory
 in lock_class

On 1/3/22 12:47, Bart Van Assche wrote:
> On 1/2/22 18:35, Waiman Long wrote:
>> -    WARN_ON_ONCE(!found);
>> -    __lockdep_free_key_range(pf, key, 1);
>> -    call_rcu_zapped(pf);
>> -    graph_unlock();
>> -out_irq:
>> +    WARN_ON_ONCE(!found && debug_locks);
>
> lockdep_unregister_key() should only be called for a registered key so 
> I'd
> like to keep the WARN_ON_ONCE(!found) here instead of changing it into
> WARN_ON_ONCE(!found && debug_locks). Otherwise this patch looks good 
> to me.

The reason for this change is to handle the case that a 
lockdep_register_key()/lockdep_unregister_key() pair may be called after 
debug_locks is turned off. Without that change, we are going to get the 
warning in lockdep_unregister_key(). That is the motivation of your 
original commit 8b39adbee805 ("locking/lockdep: Make 
lockdep_unregister_key() honor 'debug_locks' again").

Cheers,
Longman