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From:   Sven Schnelle <svens@linux.ibm.com>
To:     Breno Leitao <leitao@debian.org>
Cc:     edumazet@google.com, davem@davemloft.net, kuba@kernel.org,
        pabeni@redhat.com, kuniyu@amazon.com, netdev@vger.kernel.org,
        leit@fb.com, yoshfuji@linux-ipv6.org, dsahern@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 net-next] tcp: socket-specific version of WARN_ON_ONCE()
References: <20221208154656.60623-1-leitao@debian.org>
Date:   Sun, 11 Dec 2022 09:44:35 +0100
In-Reply-To: <20221208154656.60623-1-leitao@debian.org> (Breno Leitao's
        message of "Thu, 8 Dec 2022 07:46:56 -0800")
Message-ID: <yt9dlenes51o.fsf@linux.ibm.com>
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Breno Leitao <leitao@debian.org> writes:

> There are cases where we need relevant information about the socket
> during a warning, so, it could help us to find bugs that happens and do
> not have an easy repro.
>
> This patch creates a TCP-socket specific version of WARN_ON_ONCE(), which
> dumps revelant information about the TCP socket when it hits rare
> warnings, which is super useful for debugging purposes.
>
> Hooking this warning tcp_snd_cwnd_set() for now, but, the intent is to
> convert more TCP warnings to this helper later.
>
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  include/net/tcp.h       |  3 ++-
>  include/net/tcp_debug.h | 10 ++++++++++
>  net/ipv4/tcp.c          | 30 ++++++++++++++++++++++++++++++
>  3 files changed, 42 insertions(+), 1 deletion(-)
>  create mode 100644 include/net/tcp_debug.h
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 14d45661a84d..e490af8e6fdc 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -40,6 +40,7 @@
>  #include <net/inet_ecn.h>
>  #include <net/dst.h>
>  #include <net/mptcp.h>
> +#include <net/tcp_debug.h>
>  
>  #include <linux/seq_file.h>
>  #include <linux/memcontrol.h>
> @@ -1229,7 +1230,7 @@ static inline u32 tcp_snd_cwnd(const struct tcp_sock *tp)
>  
>  static inline void tcp_snd_cwnd_set(struct tcp_sock *tp, u32 val)
>  {
> -	WARN_ON_ONCE((int)val <= 0);
> +	TCP_SOCK_WARN_ON_ONCE(tp, (int)val <= 0);
>  	tp->snd_cwnd = val;
>  }
>  
> diff --git a/include/net/tcp_debug.h b/include/net/tcp_debug.h
> new file mode 100644
> index 000000000000..50e96d87d335
> --- /dev/null
> +++ b/include/net/tcp_debug.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_TCP_DEBUG_H
> +#define _LINUX_TCP_DEBUG_H
> +
> +void tcp_sock_warn(const struct tcp_sock *tp);
> +
> +#define TCP_SOCK_WARN_ON_ONCE(tcp_sock, condition) \
> +		DO_ONCE_LITE_IF(condition, tcp_sock_warn, tcp_sock)
> +
> +#endif  /* _LINUX_TCP_DEBUG_H */
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 54836a6b81d6..5985ba9c4231 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4705,6 +4705,36 @@ int tcp_abort(struct sock *sk, int err)
>  }
>  EXPORT_SYMBOL_GPL(tcp_abort);
>  
> +void tcp_sock_warn(const struct tcp_sock *tp)
> +{
> +	const struct sock *sk = (const struct sock *)tp;
> +	struct inet_sock *inet = inet_sk(sk);
> +	struct inet_connection_sock *icsk = inet_csk(sk);
> +
> +	WARN_ON(1);

Never looked into the details of WARN_ON, but shouldn't that come at the
end of the function? If one has kernel.panic_on_warn=1, the kernel
would already panic in WARN_ON, and the lines below wouldn't be printed?

> +
> +	pr_warn("Socket Info: family=%u state=%d ccname=%s cwnd=%u",
> +		sk->sk_family, sk->sk_state, icsk->icsk_ca_ops->name,
> +		tcp_snd_cwnd(tp));
> +
> +	switch (sk->sk_family) {
> +	case AF_INET:
> +		pr_warn("saddr=%pI4:%u daddr=%pI4:%u", &inet->inet_saddr,
> +			ntohs(inet->inet_sport), &inet->inet_daddr,
> +			ntohs(inet->inet_dport));
> +
> +		break;
> +#if IS_ENABLED(CONFIG_IPV6)
> +	case AF_INET6:
> +		pr_warn("saddr=[%pI6]:%u daddr=[%pI6]:%u", &sk->sk_v6_rcv_saddr,
> +			ntohs(inet->inet_sport), &sk->sk_v6_daddr,
> +			ntohs(inet->inet_dport));
> +		break;
> +#endif
> +	}
> +}
> +EXPORT_SYMBOL_GPL(tcp_sock_warn);
> +
>  extern struct tcp_congestion_ops tcp_reno;
>  
>  static __initdata unsigned long thash_entries;