Re: [PATCH 1/3] blk-ioc: add a new helper ioc_lookup_icq_rcu()

[Yu Kuai <yukuai@kernel.org>]

Hi,

在 2025/7/25 20:03, Jan Kara 写道:
> On Fri 25-07-25 19:21:06, Damien Le Moal wrote:
>> On 7/25/25 16:05, Yu Kuai wrote:
>>> From: Yu Kuai <yukuai3@huawei.com>
>>>
>>> ioc_lookup_icq() is used by bfq to lookup bfqq from IO path, the helper
>>> have to be protected by queue_lock, which is too heavy. Hence add a new
>>> helper that is lookless, this is safe because both request_queue and ioc
>>> can be pinged by IO that is still issuing.
>>>
>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>>> ---
>>>   block/blk-ioc.c | 34 ++++++++++++++++++++++++++++++++++
>>>   block/blk.h     |  1 +
>>>   2 files changed, 35 insertions(+)
>>>
>>> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
>>> index ce82770c72ab..4945b48dfdb6 100644
>>> --- a/block/blk-ioc.c
>>> +++ b/block/blk-ioc.c
>>> @@ -343,6 +343,40 @@ struct io_cq *ioc_lookup_icq(struct request_queue *q)
>>>   }
>>>   EXPORT_SYMBOL(ioc_lookup_icq);
>>>   
>>> +/**
>>> + * ioc_lookup_icq_rcu - lookup io_cq from ioc in io path
>>> + * @q: the associated request_queue
>>> + *
>>> + * Look up io_cq associated with @ioc - @q pair from @ioc. Must be called from
>>> + * io issue path, either return NULL if current issue io to @q for the first
>>> + * time, or return a valid icq.
>>> + */
>>> +struct io_cq *ioc_lookup_icq_rcu(struct request_queue *q)
>>> +{
>>> +	struct io_context *ioc = current->io_context;
>>> +	struct io_cq *icq;
>>> +
>>> +	WARN_ON_ONCE(percpu_ref_is_zero(&q->q_usage_counter));
>> I do not think this is necessary.
This is used to indicate this is from IO issue path, I can remove it.
>>> +
>>> +	if (!ioc)
>>> +		return NULL;
>>> +
>>> +	icq = rcu_dereference(ioc->icq_hint);
>>> +	if (icq && icq->q == q)
>>> +		return icq;
>>> +
>>> +	icq = radix_tree_lookup(&ioc->icq_tree, q->id);
>>> +	if (!icq)
>>> +		return NULL;
>>> +
>>> +	if (WARN_ON_ONCE(icq->q != q))
>>> +		return NULL;
>>> +
>>> +	rcu_assign_pointer(ioc->icq_hint, icq);
>>> +	return icq;
>>> +}
>>> +EXPORT_SYMBOL(ioc_lookup_icq_rcu);
>> Patch 2 calls this function with the rcu_read_lock() held. Why not move that rcu
>> read lock here inside this function ? That is how ioc_lookup_icq() was doing
>> things, with code that is more compact than this.
>>
>> And since ioc_lookup_icq() was already using RCU, it seems that the only change
>> you need is to remove the "lockdep_assert_held(&q->queue_lock);" from that
>> function to endup with the same above functionality. So why all the churn ?
> Yes, I agree, just dropping the assert and updating callers should be fine.
Yes, this is much simpler.
>> Another question is: is it safe to call radix_tree_lookup() without any lock
>> held ? What if this races with a radix tree insertion ? (I may be wrong here as
>> I am not familiar with that code).
> Yes, radix_tree_lookup() is fine to call with just rcu protection.

The insertion is protected by queue_lock, and look up is fine with rcu 
protection.

Thanks,
Kuai

>
> 								Honza