netdev - Re: IPv4 BUG: held lock freed!

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Date:	Sun, 19 Aug 2012 19:51:41 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Fengguang Wu <fengguang.wu@...el.com>,
	David Miller <davem@...emloft.net>,
	networking <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: IPv4 BUG: held lock freed!


	Hello,

On Sun, 19 Aug 2012, Eric Dumazet wrote:

> Hmm, this looks like sk_reset_timer() is called on a socket, and timer
> triggers _before_ the sock_hold()
> 
> So the timer handler decrements sk_refcnt to 0 and calls sk_free()
> 
> Its probably a bug introduced (or uncovered) by commit 6f458dfb40 (tcp:
> improve latencies of timer triggered events)
> 
> I always found sk_reset_timer() a bit racy...
> 
> void sk_reset_timer(struct sock *sk, struct timer_list* timer,
>                     unsigned long expires)
> {
> 	if (!mod_timer(timer, expires))
> 		sock_hold(sk); // MIGHT BE TOO LATE
> }
> 
> Following should be safer...

	Above code is fine as long as caller holds reference.

	Your change for tcp_release_cb looks correct.

	Also, may be tcp_v4_mtu_reduced is missing a check
for TCP_CLOSE state? tcp_v6_mtu_reduced already has such
check.

> void sk_reset_timer(struct sock *sk, struct timer_list* timer,
>                     unsigned long expires)
> {

	This should not be needed:

> 	sock_hold(sk);
> 	if (mod_timer(timer, expires))
> 		sock_put(sk);
> }

Regards

--
Julian Anastasov <ja@....bg>
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