Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

Waiman Long
2025-03-26T16:41:05Z

On 3/26/25 11:39 AM, Waiman Long wrote:
> On 3/26/25 1:25 AM, Boqun Feng wrote:
>>> It looks like you are trying hard to find a use case for hazard pointer in
>>> the kernel 🙂
>>>
>> Well, if it does the job, why not use it 😉 Also this shows how
>> flexible hazard pointers can be.
>>
>> At least when using hazard pointers, the reader side of the hash list
>> iteration is still lockless. Plus, since the synchronization part
>> doesn't need to wait for the RCU readers in the whole system, it will be
>> faster (I tried with the protecting-the-whole-hash-list approach as
>> well, it's the same result on the tc command). This is why I choose to
>> look into hazard pointers. Another mechanism can achieve the similar
>> behavior is SRCU, but SRCU is slightly heavier compared to hazard
>> pointers in this case (of course SRCU has more functionalities).
>>
>> We can provide a lockdep_unregister_key_nosync() without the
>> synchronize_rcu() in it and let users do the synchronization, but it's
>> going to be hard to enforce and review, especially when someone
>> refactors the code and move the free code to somewhere else.
> Providing a second API and ask callers to do the right thing is
> probably not a good idea and mistake is going to be made sooner or later.
>>> Anyway, that may work. The only problem that I see is the issue of nesting
>>> of an interrupt context on top of a task context. It is possible that the
>>> first use of a raw_spinlock may happen in an interrupt context. If the
>>> interrupt happens when the task has set the hazard pointer and iterating the
>>> hash list, the value of the hazard pointer may be overwritten. Alternatively
>>> we could have multiple slots for the hazard pointer, but that will make the
>>> code more complicated. Or we could disable interrupt before setting the
>>> hazard pointer.
>> Or we can use lockdep_recursion:
>>
>> preempt_disable();
>> lockdep_recursion_inc();
>> barrier();
>>
>> WRITE_ONCE(*hazptr, ...);
>>
>> , it should prevent the re-entrant of lockdep in irq.
> That will probably work. Or we can disable irq. I am fine with both.
>>> The solution that I am thinking about is to have a simple unfair rwlock to
>>> protect just the hash list iteration. lockdep_unregister_key() and
>>> lockdep_register_key() take the write lock with interrupt disabled. While
>>> is_dynamic_key() takes the read lock. Nesting in this case isn't a problem
>>> and we don't need RCU to protect the iteration process and so the last
>>> synchronize_rcu() call isn't needed. The level of contention should be low
>>> enough that live lock isn't an issue.
>>>
>> This could work, one thing though is that locks don't compose. Using a
>> hash write_lock in lockdep_unregister_key() will create a lockdep_lock()
>> -> "hash write_lock" dependency, and that means you cannot
>> lockdep_lock() while you're holding a hash read_lock, although it's
>> not the case today, but it certainly complicates the locking design
>> inside lockdep where there's no lockdep to help 😉
>
> Thinking about it more, doing it in a lockless way is probably a good
> idea.
>
If we are using hazard pointer for synchronization, should we also take
off "_rcu" from the list iteration/insertion/deletion macros to avoid
the confusion that RCU is being used?

Cheers,
Longman