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

Waiman Long
2025-03-26T17:02:19Z

On 3/26/25 12:47 PM, Boqun Feng wrote:
> On Wed, Mar 26, 2025 at 12:40:59PM -0400, Waiman Long wrote:
>> 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?
>>
> We can, but we probably want to introduce a new set of API with suffix
> "_lockless" or something because they will still need a lockless fashion
> similar to RCU list iteration/insertion/deletion.

The lockless part is just the iteration of the list. Insertion and
deletion is protected by lockdep_lock().

The current hlist_*_rcu() macros are doing the right things for lockless
use case too. We can either document that RCU is not being used or have
some _lockless helpers that just call the _rcu equivalent.

Cheers,
Longman