Re: [PATCH 1/1] Use RCU for the UDP hash lock

[Corey Minyard]

Stephen Hemminger wrote:
> On Thu, 25 Sep 2008 08:29:36 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
>   
> > On Wed, Sep 24, 2008 at 03:46:20PM -0500, Corey Minyard wrote:
> >     
> > > Stephen Hemminger wrote:
> > >       
> > > >   
> > > >         
> > > > >   static inline void udp_lib_unhash(struct sock *sk)
> > > > >  {
> > > > > -	write_lock_bh(&udp_hash_lock);
> > > > > -	if (sk_del_node_init(sk)) {
> > > > > +	spin_lock_bh(&udp_hash_wlock);
> > > > > +	if (sk_del_node_rcu(sk)) {
> > > > >  		inet_sk(sk)->num = 0;
> > > > >  		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> > > > >  	}
> > > > > -	write_unlock_bh(&udp_hash_lock);
> > > > > +	spin_unlock_bh(&udp_hash_wlock);
> > > > > +	synchronize_sched();
> > > > >     
> > > > >           
> > > > Could this be synchronize_rcu? You are using rcu_read_lock() protected 
> > > > sections.
> > > >   
> > > >         
> > > I meant to comment on that.  I wasn't sure which to use, so I chose the 
> > > more conservative approach.  synchronize_rcu() might be appropriate.
> > >       
> > You do indeed need to match the update-side and read-side primitives:
> >
> > 	Update-side				Read-side
> >
> > 	synchronize_rcu()			rcu_read_lock()
> > 	call_rcu()				rcu_read_unlock()
> >
> > 	call_rcu_bh()				rcu_read_lock_bh()
> > 						rcu_read_unlock_bh()
> >
> > 	synchronize_sched()			preempt_disable()
> > 						preempt_enable()
> > 						[and anything else
> > 						 that disables either
> > 						 preemption or irqs]
> >
> > 	synchronize_srcu()			srcu_read_lock()
> > 						srcu_read_unlock()
> >
> >
> > Mixing RCU or RCU-SCHED with RCU-BH will fail in Classic RCU systems,
> > while mixing RCU or RCU-BH with RCU-SCHED will fail in preemptable RCU
> > systems.  Mixing SRCU with any of the other flavors of RCU will fail
> > on any system.
> >
> > So please match them up correctly!
> >     
Ok, will do.  I read more on this, and I think I understand the issues 
better.

> >     
>
> Also, for consistency with other parts of networking code, don't introduce
> the synchronize_sched() or synchronize_srcu() pattern to network protocols
> unless there is a no other way to achieve the desired result.
>   
Do you mean synchronize_rcu(), too?  It seems to be used in the net 
code.  To avoid that I'd need to add a struct rcu_head to struct sock.  
Would that be preferable?

Thanks,

-corey
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