netdev - Re: [PATCH 1/7] xfrm: remove policy lock when accessing policy->walk.dead

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Date:	Tue, 30 Mar 2010 07:55:07 +0300
From:	Timo Teräs <timo.teras@....fi>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	netdev@...r.kernel.org
Subject: Re: [PATCH 1/7] xfrm: remove policy lock when accessing policy->walk.dead

Herbert Xu wrote:
> On Mon, Mar 29, 2010 at 05:12:38PM +0300, Timo Teras wrote:
>> @@ -1132,7 +1119,7 @@ int xfrm_sk_policy_insert(struct sock *sk, int dir, struct xfrm_policy *pol)
>>  		__xfrm_policy_link(pol, XFRM_POLICY_MAX+dir);
>>  	}
>>  	if (old_pol)
>> -		__xfrm_policy_unlink(old_pol, XFRM_POLICY_MAX+dir);
>> +		old_pol = __xfrm_policy_unlink(old_pol, XFRM_POLICY_MAX+dir);
>>  	write_unlock_bh(&xfrm_policy_lock);
>>  
>>  	if (old_pol) {
> 
> So when can this actually fail?

Considering that the socket reference is received from the sk->sk_policy,
and the hash bucket we use is "XFRM_POLICY_MAX+dir", it's non-obvious if
it can fail or not.

It would look like the timer can kill a policy and unlink it, but it
would still be found from sk_policy.

It probably doesn't really make sense to insert per-socket policy that
expires. But in case someone does something like that, I'd think we
need the above just to be sure.

Considering this, xfrm_sk_policy_lookup() should probably check the
dead flag, and cleanup sk_policy if it was killed by a timer.
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