linux-kernel - Re: [PATCH RFC] hlist_add_tail

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Date:   Mon, 5 Dec 2016 21:37:43 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Miller <davem@...emloft.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] hlist_add_tail_rcu disable sparse warning

On Wed, Nov 23, 2016 at 10:48:19PM +0200, Michael S. Tsirkin wrote:
> sparse is unhappy about this code in hlist_add_tail_rcu:
> 
>         struct hlist_node *i, *last = NULL;
> 
>         for (i = hlist_first_rcu(h); i; i = hlist_next_rcu(i))
>                 last = i;
> 
> This is because hlist_next_rcu and hlist_next_rcu return
> __rcu pointers.
> 
> The following trivial patch disables the warning
> without changing the behaviour in any way.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

So after reviewing this, there's no rcu-ness involved
except when we assign the next pointer on the last item.
This is because as the following comment says
 * The caller must take whatever precautions are necessary
 * (such as holding appropriate locks) to avoid racing
 * with another list-mutation primitive, such as hlist_add_head_rcu()
 * or hlist_del_rcu(), running on this same list.

I conclude this patch is actually the right thing to do, by comparison,
__hlist_for_each_rcu suggested by Dave Miller would be confusing since
it's designed to run in the rcu read side critical section.

I'll repost as non-RFC unless I hear otherwise.


> ---
> 
> I would appreciate review to confirm the function doesn't
> do anything unsafe though.
> 
> In particular, should this use __hlist_for_each_rcu instead?
> I note that __hlist_for_each_rcu does rcu_dereference
> internally, which is missing here.
> 
> compile-tested only.
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 8beb98d..33574db 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -511,7 +511,7 @@ static inline void hlist_add_tail_rcu(struct hlist_node *n,
>  {
>  	struct hlist_node *i, *last = NULL;
>  
> -	for (i = hlist_first_rcu(h); i; i = hlist_next_rcu(i))
> +	for (i = h->first; i; i = i->next)
>  		last = i;
>  
>  	if (last) {