Date: Wed, 19 Dec 2012 18:02:12 -0500 From: Karim Fodil-Lemelin <fodillemlinkarim@gmail.com> To: Vijay Singh <vijju.singh@gmail.com> Cc: freebsd-net@freebsd.org, Navdeep Parhar <nparhar@gmail.com> Subject: Re: use of V_tcbinfo lock for TCP syncache Message-ID: <50D24774.80800@gmail.com> In-Reply-To: <CALCNsJS8j0wvv4dc1Q6rLckmr0RE%2BgW0%2BmUDdmSvyfjXzKugsQ@mail.gmail.com> References: <CALCNsJRFyQ9ZmfJ3quX2-cUuFHjs2rXw63Tq5ZH-uP1%2BoQmjLw@mail.gmail.com> <50D218BA.7080301@FreeBSD.org> <50D21C3B.8020803@gmail.com> <CALCNsJS8j0wvv4dc1Q6rLckmr0RE%2BgW0%2BmUDdmSvyfjXzKugsQ@mail.gmail.com>
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On 19/12/2012 4:01 PM, Vijay Singh wrote:
>>> Holding the pcbinfo lock prevents races between syncache_socket() and
>>> accept(). See rwatson's comment just above tcp_usr_accept. I noted
>>> this too (the comment above tod->tod_offload_socket() in tcp_syncache.c)
>>> back when I updated the TOE code in the kernel.
>> er, I think I told you why tcp_usr_accept holds the pcbinfo lock, which
>> wasn't your original question... :-)
> But it helped.
>
> So I am thinking about trying a change where syncache_socket() would
> call soalloc() first, get a socket, setup the inp, and then do a
> (modified) sonewconn to place the socket in the listener's queue.
> Robert's comment indicated that this would be a better way to
> eliminate the race since we wouldn't need the pcblock when we make the
> sonewconn call.
>
>
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 one.
Karim.
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