netdev - Re: [PATCH] inetpeer: optimizations

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Date:	Sun, 06 Dec 2009 19:58:07 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] inetpeer: optimizations

Jarek Poplawski a écrit :
> On Sun, Dec 06, 2009 at 09:47:03AM +0100, Eric Dumazet wrote:
>> Jarek Poplawski a écrit :
>>> Eric Dumazet wrote, On 12/05/2009 01:11 PM:
>>>
>>>> - Use atomic_dec_and_test() in inet_putpeer()
>>> atomic_dec_and_lock()?
>> Yes :)
>>
>>>>   This takes/dirties the lock only if necessary.
>>> ...
>>>
>>>>  void inet_putpeer(struct inet_peer *p)
>>>>  {
>>>> -	spin_lock_bh(&inet_peer_unused_lock);
>>>> -	if (atomic_dec_and_test(&p->refcnt)) {
>>>> -		list_add_tail(&p->unused, &unused_peers);
>>>> +	local_bh_disable();
>>>> +	if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
>>> Why not:
>>> 	if (atomic_dec_and_test(&p->refcnt)) {
>>> 		spin_lock_bh(&inet_peer_unused_lock);
>>> 		...
>> Because we have to take the lock before doing the final 1 -> 0 refcount transition.
>>
>> (Another thread could do the 0 -> 1 transition)
> 
> AFAICS this lock here can only to prevent double linking to the
> unused_peers list during such transitions. If so, it could be replaced
> with the list_empty(&p->unused) test before list_add_tail(), and
> atomic_dec_test() without the lock would be enough (unless I miss
> something ;-).
> 

Yes, you miss something. We are not working on a true reference count variable.
(p is referenced in avl tree but there is no +1 count for this reference)

Its more a usecount one, and p usecount can be 0 but p still in avl tree.

Even if we are the thread (A) doing 1 -> 0 transition, other thread (B)
can find p and perform the opposite 0 -> 1 transition.

If (B) tries to unlink p before (A), it finds p already unlinked.

Then (A) links into unused list, while refcnt is still 1

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