netdev - Re: a problem tcp_v4

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Message-ID: <20101112182959.GA20459@ms2.inr.ac.ru>
Date:	Fri, 12 Nov 2010 21:29:59 +0300
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	David Lamparter <equinox@...c24.net>,
	Eric Paris <eparis@...hat.com>, Hua Zhong <hzhong@...il.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, pekkas@...core.fi, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, paul.moore@...com
Subject: Re: a problem tcp_v4_err()

Hello!

On Fri, Nov 12, 2010 at 07:12:58PM +0100, Eric Dumazet wrote:
> I see socket is locked around line 368,
> 
>         bh_lock_sock(sk);
>         /* If too many ICMPs get dropped on busy
>          * servers this needs to be solved differently.
>          */
>         if (sock_owned_by_user(sk))
>                 NET_INC_STATS_BH(net, LINUX_MIB_LOCKDROPPEDICMPS);
> 
> 
> Hmm, maybe some goto is missing ;)

It is not missing, sock_owned_by_user() is checked later when some operation which
cannot be done without lock is required. It was done to save error in sk_err_soft even
when socket is locked.

This code also _understands_ this: look at

                } else if (sock_owned_by_user(sk)) {
                        /* RTO revert clocked out retransmission,
                         * but socket is locked. Will defer. */
                        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                                  HZ/20, TCP_RTO_MAX);

but somehow it considers the manipulations with rto/backoff/write_queue as safe.
Seems, they are not.

Alexey



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