linux-kernel - Re: [PATCH 8/8] fs/aio: Use rcu_work instead of explicit rcu and work item

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Date:   Wed, 21 Mar 2018 18:17:43 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     torvalds@...ux-foundation.org, jannh@...gle.com,
        paulmck@...ux.vnet.ibm.com, bcrl@...ck.org,
        viro@...iv.linux.org.uk, kent.overstreet@...il.com,
        security@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 8/8] fs/aio: Use rcu_work instead of explicit rcu and
 work item

On 03/21, Tejun Heo wrote:
>
> Hey, Oleg.
>
> On Wed, Mar 21, 2018 at 04:58:13PM +0100, Oleg Nesterov wrote:
> > > -	struct rcu_head		free_rcu;
> > > -	struct work_struct	free_work;	/* see free_ioctx() */
> > > +	struct rcu_work		free_rwork;	/* see free_ioctx() */
> >
> > IIUC, you can't easily share rcu_work's, thus every kioctx needs its own
> > ->free_rwork and this looks sub-optimal.
> >
> > What do you think about the (untested) patch below?
> >
> > Oleg.
> >
> >
> > --- a/fs/aio.c
> > +++ b/fs/aio.c
> > @@ -115,8 +115,10 @@ struct kioctx {
> >  	struct page		**ring_pages;
> >  	long			nr_pages;
> >
> > -	struct rcu_head		free_rcu;
> > -	struct work_struct	free_work;	/* see free_ioctx() */
> > +	union {
> > +		struct rcu_head		free_rcu;
> > +		struct llist_node	free_llist;
> > +	};
>
> It really depends on how much we want to optimize.  Do you think it
> matters enough?

I have no idea, probably not.

Mostly I am asking because I do not really understand
"[PATCH 6/8] RCU, workqueue: Implement rcu_work".

I mean, the code looks simple and correct but why does it play with
WORK_STRUCT_PENDING_BIT? IOW, I do not see a "good" use-case when 2 or more
queue_rcu_work()'s can use the same rwork and hit work_pending() == T. And
what the caller should do if queue_rcu_work() returns false?

Oleg.