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

[Yu Kuai]

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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > >   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