use of V_tcbinfo lock for TCP syncache

[Vijay Singh]

> Sure but syncache_expand() is entered with the tcbinfo already write locked
> which also protects the unlocking of the listening connection and the
> locking of the newly created socket. Around this part:
>
>             /*
>              * Socket is created in state SYN_RECEIVED.
>              * Unlock the listen socket, lock the newly
>              * created socket and update the tp variable.
>              */
>             INP_WUNLOCK(inp);    /* listen socket */
>             inp = sotoinpcb(so);
>             INP_WLOCK(inp);        /* new connection */
>             tp = intotcpcb(inp);
>
>  Without the tcbinfo lock the new socket could be closed (getting a reset)
> which would put it in INP_TIMEWAIT or INP_DROPPED _after_ the check is made
> in tcp_usr_accept since there is a period of time where tcbinfo is not
> locked and the new socket inp is not locked either.
>
> I could be wrong but it seems that without the tcbinfo lock a lot could
> happen between the unlocking of the listen socket and the locking of the new

Hopefully Robert will chime in.

I am sorry that I was not clear. In my experiment, syncache_expand()
is still entered with the V_tcbinfo lock held.

In my (limited) view, sonewconn() is overleaded. It

[1] allocates a new socket
[2] initializes it using the listener socket
[3] invokes the pru_attach routine, where the inp is allocated
[4] it inserts the socket in the listeners queue
[5] it, optionally, notifies the listener of the new connection

When sonewconn returns, we do a bunch of things, [6] such as call
in_pcbconnect() and set state in tp etc.

What I am experimenting with is to separate out [4] & [5] from the
list above, and move those to AFTER we do the inp processing in [6].
At that point I do not think that the pcbinfo lock should be required
to be held.

-vijay