linux-kernel - Re: [PATCH 1/3] blk-ioc: add a new helper ioc_lookup_icq

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Message-ID: <3up6wgkarspq7zo34pe72zd5a5lygdo2sokbstxc63fajrl3gw@tpk3ihmc7k7l>
Date: Fri, 25 Jul 2025 14:03:46 +0200
From: Jan Kara <jack@...e.cz>
To: Damien Le Moal <dlemoal@...nel.org>
Cc: Yu Kuai <yukuai1@...weicloud.com>, jack@...e.cz, axboe@...nel.dk, 
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, yukuai3@...wei.com, 
	yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH 1/3] blk-ioc: add a new helper ioc_lookup_icq_rcu()

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@...wei.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@...wei.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.
> 
> > +
> > +	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.

> 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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR