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

From: Stephen Hemminger <[email-masked]>
Date: Thu, 25 Sep 2008 08:34:31 -0700

On Thu, 25 Sep 2008 08:29:36 -0700
"Paul E. McKenney" <[email-masked]> 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!
> 

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.