netdev - Re: [PATCH] inetpeer: optimizations

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Date:	Sun, 6 Dec 2009 15:18:05 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] inetpeer: optimizations

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:
> >>  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)
> 
> I'll cook a followup patch to also avoid taking the lock in the  1+ -> 2+ transitions.

I see... So it's this concept of atomic refcounts with locking, which
I can't get used to. Anyway, since local_bh_disable/enable() are more
than one or two asm instructions, and this all is about optimization,
it seems to me it's worth to avoid it with one of these:
a) additional atomic test under the lock after unlocked
   atomic_dec_and_test(),
b) implementing atomic_dec_and_lock_bh(),
c) if there are are problems with b), open code it here.

Thanks,
Jarek P.
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