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Message-ID: <4A2F6F39.8060107@gmail.com>
Date: Wed, 10 Jun 2009 10:30:49 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: David Miller <davem@...emloft.net>
CC: rusty@...tcorp.com.au, netdev@...r.kernel.org
Subject: Re: [PATCH] net: No more expensive sock_hold()/sock_put() on each
tx
David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Thu, 04 Jun 2009 11:18:35 +0200
>
>> @@ -1172,12 +1186,18 @@ void __init sk_init(void)
>> void sock_wfree(struct sk_buff *skb)
>> {
>> struct sock *sk = skb->sk;
>> + int res;
>>
>> /* In case it might be waiting for more memory. */
>> - atomic_sub(skb->truesize, &sk->sk_wmem_alloc);
>> + res = atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc);
>> if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE))
>> sk->sk_write_space(sk);
>> - sock_put(sk);
>> + /*
>> + * if sk_wmem_alloc reached 0, we are last user and should
>> + * free this sock, as sk_free() call could not do it.
>> + */
>> + if (res == 0)
>> + __sk_free(sk);
>> }
>> EXPORT_SYMBOL(sock_wfree);
>>
>
> Eric, I don't understand this part, please enlighten me :-)
>
> Just because we've liberated all of the write buffer space, that does
> not mean that it's time to kill off the socket completely.
>
> Right?
Remember we initialize this field to one.
If we freed all write buffer space, final value is one, not zero.
res == 0 only if we both freed all write buffer space, *and* socket was
also refcounted to 0 (sk_free() then realized it could not yet call __sk_free())
So we cheat a litle bit, because of this offset of one, we might block a sender a litle bit earlier :)
Thank you
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