Subject: Re: [PATCH 3/3] net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls

From: Eric Dumazet <[email-masked]>
Date: Thu, 13 Nov 2008 14:51:49 +0100

Peter Zijlstra a écrit :
> On Thu, 2008-11-13 at 14:15 +0100, Eric Dumazet wrote:
>> +begin:
>> +       sk_nulls_for_each_rcu(sk, node, &head->chain) {
>>                 if (INET_MATCH(sk, net, hash, acookie,
>> +                                       saddr, daddr, ports, dif)) {
>> +                       if (unlikely(!atomic_inc_not_zero(&sk->sk_refcnt)))
>> +                               goto begintw;
>> +                       if (unlikely(!INET_MATCH(sk, net, hash, acookie,
>> +                               saddr, daddr, ports, dif))) {
>> +                               sock_put(sk);
>> +                               goto begin;
>> +                       }
> 
> This is the validation step that verifies the race opened by using
> SLAB_DESTROY_BY_RCU, right?

The atomic_inc_not_zero() is not related to SLAB_DESTROY_BY_RCU but
classic RCU lookup. A writer can delete the item right before we try to use it.

Next step is necessary in case the deleted item was re-allocated and inserted
in a hash chain (this one or another one, it doesnt matter). In this case,
previous atomic_inc_not_zero test will succeed. So we must check again the item
we selected (and refcounted) is the one we were searching.

So yes, this bit should be documented, since SLAB_DESTROY_BY_RCU is
not really used in linux kernel at this moment.

> 
> Does it make sense to add a little comment to these validation steps to
> keep people on their toes and aware of the trickery?

Yes, you are right.

Thanks