netdev - Re: [PATCH] tcp: Pass lockdep expression to RCU lists

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Date:   Wed, 19 Feb 2020 11:35:21 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Amol Grover <frextrite@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: Pass lockdep expression to RCU lists



On 2/19/20 2:05 AM, Amol Grover wrote:
> tcp_cong_list is traversed using list_for_each_entry_rcu
> outside an RCU read-side critical section but under the protection
> of tcp_cong_list_lock.
>

This is not true.

There are cases where RCU read lock is held,
and others where the tcp_cong_list_lock is held.

I believe you need to be more precise in the changelog.

If there was a bug, net tree would be the target for this patch,
with a required Fixes: tag.

Otherwise, if net-next tree is the intended target, you have to signal
it, as instructed in Documentation/networking/netdev-FAQ.rst

Thanks.

 
> Hence, add corresponding lockdep expression to silence false-positive
> warnings, and harden RCU lists.
> 
> Signed-off-by: Amol Grover <frextrite@...il.com>
> ---
>  net/ipv4/tcp_cong.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
> index 3737ec096650..8d4446ed309e 100644
> --- a/net/ipv4/tcp_cong.c
> +++ b/net/ipv4/tcp_cong.c
> @@ -25,7 +25,8 @@ static struct tcp_congestion_ops *tcp_ca_find(const char *name)
>  {
>  	struct tcp_congestion_ops *e;
>  
> -	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
> +	list_for_each_entry_rcu(e, &tcp_cong_list, list,
> +				lockdep_is_held(&tcp_cong_list_lock)) {
>  		if (strcmp(e->name, name) == 0)
>  			return e;
>  	}
> @@ -55,7 +56,8 @@ struct tcp_congestion_ops *tcp_ca_find_key(u32 key)
>  {
>  	struct tcp_congestion_ops *e;
>  
> -	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
> +	list_for_each_entry_rcu(e, &tcp_cong_list, list,
> +				lockdep_is_held(&tcp_cong_list_lock)) {
>  		if (e->key == key)
>  			return e;
>  	}
> @@ -317,7 +319,8 @@ int tcp_set_allowed_congestion_control(char *val)
>  	}
>  
>  	/* pass 2 clear old values */
> -	list_for_each_entry_rcu(ca, &tcp_cong_list, list)
> +	list_for_each_entry_rcu(ca, &tcp_cong_list, list,
> +				lockdep_is_held(&tcp_cong_list_lock))
>  		ca->flags &= ~TCP_CONG_NON_RESTRICTED;
>  
>  	/* pass 3 mark as allowed */
>