linux-kernel - Re: [PATCH v2] hlist_add_tail

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Date:   Wed, 1 Mar 2017 11:37:53 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2] hlist_add_tail_rcu disable sparse warning

On Mon, Feb 27, 2017 at 09:14:19PM +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.
> 
> It's a false positive - it's a write side primitive and so
> does not need to be called in a read side critical section.
> 
> The following trivial patch disables the warning
> without changing the behaviour in any way.
> 
> Note: __hlist_for_each_rcu would also remove the warning but it would be
> confusing since it calls rcu_derefence and is designed to run in the rcu
> read side critical section.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Queud for further review and testing, thank you both!

							Thanx, Paul

> ---
> 
> Comments from v2:
> 	add a comment as requested by Steven Rostedt
> 
>  include/linux/rculist.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 4f7a956..b1fd8bf 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -509,7 +509,8 @@ 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))
> +	/* Note: write side code, so rcu accessors are not needed. */
> +	for (i = h->first; i; i = i->next)
>  		last = i;
> 
>  	if (last) {
> -- 
> MST
>