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

Stephen Hemminger
2008-09-25T15:34:54Z

On Thu, 25 Sep 2008 08:29:36 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> 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.