linux-kernel - Re: [PATCH 4.19 012/120] tcp: annotate tp->write

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Date:   Tue, 16 Mar 2021 10:50:49 +0100
From:   Pavel Machek <pavel@...x.de>
To:     gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 4.19 012/120] tcp: annotate tp->write_seq lockless reads

Hi!

> From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> From: Eric Dumazet <edumazet@...gle.com>

Dup.


> We need to add READ_ONCE() annotations, and also make
> sure write sides use corresponding WRITE_ONCE() to avoid
> store-tearing.

> @@ -1037,7 +1037,7 @@ new_segment:
>  		sk->sk_wmem_queued += copy;
>  		sk_mem_charge(sk, copy);
>  		skb->ip_summed = CHECKSUM_PARTIAL;
> -		tp->write_seq += copy;
> +		WRITE_ONCE(tp->write_seq, tp->write_seq + copy);
>  		TCP_SKB_CB(skb)->end_seq += copy;
>  		tcp_skb_pcount_set(skb, 0);
>

I wonder if this needs to do READ_ONCE, too?

> @@ -1391,7 +1391,7 @@ new_segment:
>  		if (!copied)
>  			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH;
>  
> -		tp->write_seq += copy;
> +		WRITE_ONCE(tp->write_seq, tp->write_seq + copy);
>  		TCP_SKB_CB(skb)->end_seq += copy;
>  		tcp_skb_pcount_set(skb, 0);
>

And here.

> @@ -2593,9 +2594,12 @@ int tcp_disconnect(struct sock *sk, int
>  	sock_reset_flag(sk, SOCK_DONE);
>  	tp->srtt_us = 0;
>  	tp->rcv_rtt_last_tsecr = 0;
> -	tp->write_seq += tp->max_window + 2;
> -	if (tp->write_seq == 0)
> -		tp->write_seq = 1;
> +
> +	seq = tp->write_seq + tp->max_window + 2;
> +	if (!seq)
> +		seq = 1;
> +	WRITE_ONCE(tp->write_seq, seq);

And here.

> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -510,7 +510,7 @@ struct sock *tcp_create_openreq_child(co
>  	newtp->app_limited = ~0U;
>  
>  	tcp_init_xmit_timers(newsk);
> -	newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
> +	WRITE_ONCE(newtp->write_seq, newtp->pushed_seq = treq->snt_isn + 1);

Would it be better to do assignment to pushed_seq outside of
WRITE_ONCE macro? This is ... "interesting".

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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