netdev - Re: [PATCH v3] net-tcp: TCP/IP stack bypass for loopback connections

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Message-ID: <CAEkNxbGbR_CGOo=npnBLtCq_yQr4G8CGzkbk0CJ7BRw951nWQQ@mail.gmail.com>
Date:	Mon, 17 Sep 2012 16:08:25 -0700
From:	Bruce Curtis <brutus@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v3] net-tcp: TCP/IP stack bypass for loopback connections

On Mon, Sep 17, 2012 at 1:20 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> On Mon, 2012-09-17 at 11:58 -0700, Bruce "Brutus" Curtis wrote:
> > From: "Bruce \"Brutus\" Curtis" <brutus@...gle.com>
> >
> > TCP/IP loopback socket pair stack bypass, based on an idea by, and
> > rough upstream patch from, David Miller <davem@...emloft.net> called
> > "friends", the data structure modifcations and connection scheme are
> > reused with extensive data-path changes.
>
> ...
>
> >
> > +             if (skb->friend) {
> > +                     /*
> > +                      * If friends haven't been made yet, our sk_friend
> > +                      * still == NULL, then update with the ACK's friend
> > +                      * value (the listen()er's sock addr) which is used
> > +                      * as a place holder.
> > +                      */
> > +                     cmpxchg(&sk->sk_friend, NULL, skb->friend);
> > +             }
>
>
> There is a fundamental issue with this patch
>
> Setting skb->friend to a socket structure, without holding a reference
> on it is going to add subtle races and bugs.
>
> In this code, we have no guarantee the socket pointed by skb->friend was
> eventually freed and/or reused.
>
> But adding references might be overkill, as we need to unref them in
> some places, in hot path.

Yes, this could be a problem, putting a hold on the sk would be overkill for the
hot-path, working on it now, more soon.
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