linux-kernel - Re: [PATCH] hlist_add_tail

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Date:   Wed, 26 Apr 2017 07:02:57 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.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 Wed, Apr 26, 2017 at 09:13:43AM -0400, Steven Rostedt wrote:
> 
> Paul,
> 
> Did you see this email?

Yep! Michael's patch is 48ac34666ff7 ("hlist_add_tail_rcu disable sparse
warning") in -rcu and now in -tip.

But I do appreciate the reminder -- way too easy to miss stuff on LKML!

							Thanx, Paul

> -- Steve
> 
> 
> On Mon, 27 Feb 2017 20:26:01 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > 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  
>