netdev - Re: [PATCH net-next 1/2] tcp/dccp: Move initialisation of refcounted

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Date:   Tue, 24 Mar 2020 20:10:13 +0900
From:   Kuniyuki Iwashima <kuniyu@...zon.co.jp>
To:     <edumazet@...gle.com>
CC:     <davem@...emloft.net>, <dccp@...r.kernel.org>,
        <gerrit@....abdn.ac.uk>, <kuba@...nel.org>, <kuni1840@...il.com>,
        <kuniyu@...zon.co.jp>, <kuznet@....inr.ac.ru>,
        <netdev@...r.kernel.org>, <osa-contribution-log@...zon.com>,
        <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH net-next 1/2] tcp/dccp: Move initialisation of refcounted

From:   Eric Dumazet <edumazet@...gle.com>
Date:   Mon, 23 Mar 2020 11:45:48 -0700
> On Mon, Mar 23, 2020 at 11:18 AM Kuniyuki Iwashima <kuniyu@...zon.co.jp> wrote:
> >
> > The refcounted is initialised at most three times.
> >
> >   - __inet_lookup_skb() sets it true.
> >   - skb_steal_sock() is false and __inet_lookup() sets it true.
> >   - __inet_lookup_established() is false and __inet_lookup() sets it false.
> >
> > We do not need to initialise refcounted again and again, so we should do
> > it just before return.
> >
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> > ---
> 
> Well, I do not believe this patch (and the following one) makes things
> more readable.
> 
> I doubt setting a boolean in a register or a stack variable has any cost,
> I prefer letting the compiler optimize this.
> 
> The ehash lookup cost is at least 2 or 3 cache lines, this is the
> major contribution.

I confirmed that GCC (v7.3.1) does not make any change with this patch,
but the original code is bit confusing. I think this patch makes the code
bit more readable so that someone (like me...) will not doubt about
complier optimization and write patches.

I will respin only this patch because the changelog can be incorrect by
complier optimization.

Thanks.