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

Paul E. McKenney
2008-09-25T15:30:08Z

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!

Thanx, Paul