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Sender: Navdeep Parhar <nparhar@gmail.com>
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Date: Wed, 19 Dec 2012 11:42:50 -0800
From: Navdeep Parhar <np@FreeBSD.org>
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To: Vijay Singh <vijju.singh@gmail.com>
Subject: Re: use of V_tcbinfo lock for TCP syncache
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On 12/19/12 11:31, Vijay Singh wrote:
> As it is today, a socket upcall on a listener socket is made with the
> V_tcbinfo lock held. [tcp_input -> syncache_socket -> sonewconn ->
> sowakeup].
> 
> I feel that the use of the V_tcbinfo is not consistent in the syncache code.
> 
> In syncache_add(), we drop the lock before doing the lookup:
> 
>         INP_WUNLOCK(inp);
>         INP_INFO_WUNLOCK(&V_tcbinfo);
>         [..snip..]
>         sc = syncache_lookup(inc, &sch);        /* returns locked entry */
> 
> However, when going through syncache_expand() or syncache_chkrst() we
> keep the V_tcbinfo lock.
> 
> Since the syncache has its own lock, do we need to hold the V_tcbinfo
> lock when calling syncache_socket()?

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.

Regards,
Navdeep