netdev - Re: [PATCH] tcp: tcp_release_cb() should release socket ownership

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Message-Id: <20140305.205938.287751582220842351.davem@davemloft.net>
Date:	Wed, 05 Mar 2014 20:59:38 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	lars.persson@...s.com, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: tcp_release_cb() should release socket ownership

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 04 Mar 2014 14:39:12 -0800

> @@ -767,6 +767,17 @@ void tcp_release_cb(struct sock *sk)
>  	if (flags & (1UL << TCP_TSQ_DEFERRED))
>  		tcp_tsq_handler(sk);
>  
> +	/* Here begins the tricky part :
> +	 * We are called from release_sock() with :
> +	 * 1) BH disabled
> +	 * 2) sk_lock.slock spinlock held
> +	 * 3) socket owned by us (sk->sk_lock.owned == 1)
> +	 *
> +	 * But following code is meant to be called from BH handlers,
> +	 * so we should keep BH disabled, but early release socket ownership
> +	 */
> +	sock_release_ownership(sk);
> +

It really means that sk_lock.owned cannot ever be accessed without the
sk_lock spinlock held.

Most of this is easy to hand audit, except sock_owned_by_user() which
has call sites everywhere.

Consider adding a locking assertion to it.
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