netdev - Re: [PATCH net-next 1/4] scsi_tcp: block BH in TCP callbacks

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Message-ID: <573CA47F.10304@cs.wisc.edu>
Date:	Wed, 18 May 2016 12:21:03 -0500
From:	Mike Christie <michaelc@...wisc.edu>
To:	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Venkatesh Srinivas <venkateshs@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 1/4] scsi_tcp: block BH in TCP callbacks

On 05/17/2016 07:44 PM, Eric Dumazet wrote:
> iscsi_sw_tcp_data_ready() and iscsi_sw_tcp_state_change() were
> using read_lock(&sk->sk_callback_lock) which is fine if caller
> disabled BH.
> 
> TCP stack no longer has this requirement and can run from
> process context.
> 
> Use read_lock_bh() variant to restore previous assumption.
> 
> Ideally this code could use RCU instead...
> 
> Fixes: 5413d1babe8f ("net: do not block BH while processing socket backlog")
> Fixes: d41a69f1d390 ("tcp: make tcp_sendmsg() aware of socket backlog")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Mike Christie <michaelc@...wisc.edu>
> Cc: Venkatesh Srinivas <venkateshs@...gle.com>
> ---
>  drivers/scsi/iscsi_tcp.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/scsi/iscsi_tcp.c b/drivers/scsi/iscsi_tcp.c
> index 2e4c82f8329c..ace4f1f41b8e 100644
> --- a/drivers/scsi/iscsi_tcp.c
> +++ b/drivers/scsi/iscsi_tcp.c
> @@ -131,10 +131,10 @@ static void iscsi_sw_tcp_data_ready(struct sock *sk)
>  	struct iscsi_tcp_conn *tcp_conn;
>  	read_descriptor_t rd_desc;
>  
> -	read_lock(&sk->sk_callback_lock);
> +	read_lock_bh(&sk->sk_callback_lock);
>  	conn = sk->sk_user_data;
>  	if (!conn) {
> -		read_unlock(&sk->sk_callback_lock);
> +		read_unlock_bh(&sk->sk_callback_lock);
>  		return;
>  	}
>  	tcp_conn = conn->dd_data;
> @@ -154,7 +154,7 @@ static void iscsi_sw_tcp_data_ready(struct sock *sk)
>  	/* If we had to (atomically) map a highmem page,
>  	 * unmap it now. */
>  	iscsi_tcp_segment_unmap(&tcp_conn->in.segment);
> -	read_unlock(&sk->sk_callback_lock);
> +	read_unlock_bh(&sk->sk_callback_lock);
>  }
>  
>  static void iscsi_sw_tcp_state_change(struct sock *sk)
> @@ -165,10 +165,10 @@ static void iscsi_sw_tcp_state_change(struct sock *sk)
>  	struct iscsi_session *session;
>  	void (*old_state_change)(struct sock *);
>  
> -	read_lock(&sk->sk_callback_lock);
> +	read_lock_bh(&sk->sk_callback_lock);
>  	conn = sk->sk_user_data;
>  	if (!conn) {
> -		read_unlock(&sk->sk_callback_lock);
> +		read_unlock_bh(&sk->sk_callback_lock);
>  		return;
>  	}
>  	session = conn->session;
> @@ -179,7 +179,7 @@ static void iscsi_sw_tcp_state_change(struct sock *sk)
>  	tcp_sw_conn = tcp_conn->dd_data;
>  	old_state_change = tcp_sw_conn->old_state_change;
>  
> -	read_unlock(&sk->sk_callback_lock);
> +	read_unlock_bh(&sk->sk_callback_lock);
>  
>  	old_state_change(sk);
>  }

Can I just confirm that nested bh lock calls like:

spin_lock_bh(lock1);
spin_lock_bh(lock2);

do something

spin_unlock_bh(lock2);
spin_unlock_bh(lock1);

is ok? It seems smatch sometimes warns about this.

I found this thread

https://lkml.org/lkml/2011/1/25/232

which says it is ok.