Subject: Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 27 Mar 2014 14:28:31 -0700

On Mon, Mar 24, 2014 at 07:11:44AM -0400, Neil Horman wrote:
> On Mon, Mar 24, 2014 at 12:42:46AM +0530, Monam Agarwal wrote:
> > This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> > 
> > The rcu_assign_pointer() ensures that the initialization of a structure       
> > is carried out before storing a pointer to that structure. 
> > And in the case of the NULL pointer, there is no structure to initialize. 
> > So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> > 
> > Signed-off-by: Monam Agarwal <[email-masked]>
> No, I don't think this is right. __netpoll_cleanup can be called in parallel
> with bottom half access to dev->npinfo.  rcu_assign pointer is safe against
> those accesses, but RCU_INIT_POINTER is not.

Hello, Neil,

But in this case, NULL is being assigned, so there should be no problem.
If this was a non-NULL pointer to a structure that had not already been
exposed to readers, then yes, this would be a problem.  But NULL pointers
are OK to assign using RCU_INIT_POINTER().

							Thanx, Paul

> Neil
> 
> > ---
> >  net/core/netpoll.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> > index 7291dde..41c4e9c 100644
> > --- a/net/core/netpoll.c
> > +++ b/net/core/netpoll.c
> > @@ -809,7 +809,7 @@ void __netpoll_cleanup(struct netpoll *np)
> >  		if (ops->ndo_netpoll_cleanup)
> >  			ops->ndo_netpoll_cleanup(np->dev);
> >  
> > -		rcu_assign_pointer(np->dev->npinfo, NULL);
> > +		RCU_INIT_POINTER(np->dev->npinfo, NULL);
> >  		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
> >  	}
> >  }
> > -- 
> > 1.7.9.5
> > 
> > 
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