linux-kernel - Re: [PATCH] hlist_add_tail

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Message-ID: <20170227202547-mutt-send-email-mst@kernel.org>
Date:   Mon, 27 Feb 2017 20:26:01 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH] hlist_add_tail_rcu disable sparse warning

On Fri, Feb 10, 2017 at 07:39:49PM +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>
> ---

ping

> changes since RFC
> 	added commit log text to explain why don't we use __hlist_for_each_rcu
> 
>  include/linux/rculist.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 4f7a956..bf578e8 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -509,7 +509,7 @@ 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) {
> -- 
> MST