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Date:   Wed, 15 Jul 2020 13:21:41 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     John Ogness <john.ogness@...utronix.de>,
        Willem de Bruijn <willemb@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] af_packet: TPACKET_V3: replace busy-wait loop

On Tue,  7 Jul 2020 17:28:04 +0206 John Ogness wrote:
> A busy-wait loop is used to implement waiting for bits to be copied
> from the skb to the kernel buffer before retiring a block. This is
> a problem on PREEMPT_RT because the copying task could be preempted
> by the busy-waiting task and thus live lock in the busy-wait loop.
> 
> Replace the busy-wait logic with an rwlock_t. This provides lockdep
> coverage and makes the code RT ready.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Is taking a lock and immediately releasing it better than a completion?
Seems like the lock is guaranteed to dirty a cache line, which would
otherwise be avoided here.

Willem, would you be able to take a look as well? Is this path
performance sensitive in real life?

> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 29bd405adbbd..dd1eec2dd6ef 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -593,6 +593,7 @@ static void init_prb_bdqc(struct packet_sock *po,
>  						req_u->req3.tp_block_size);
>  	p1->tov_in_jiffies = msecs_to_jiffies(p1->retire_blk_tov);
>  	p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
> +	rwlock_init(&p1->blk_fill_in_prog_lock);
>  
>  	p1->max_frame_len = p1->kblk_size - BLK_PLUS_PRIV(p1->blk_sizeof_priv);
>  	prb_init_ft_ops(p1, req_u);
> @@ -659,10 +660,9 @@ static void prb_retire_rx_blk_timer_expired(struct timer_list *t)
>  	 *
>  	 */
>  	if (BLOCK_NUM_PKTS(pbd)) {
> -		while (atomic_read(&pkc->blk_fill_in_prog)) {
> -			/* Waiting for skb_copy_bits to finish... */
> -			cpu_relax();
> -		}
> +		/* Waiting for skb_copy_bits to finish... */
> +		write_lock(&pkc->blk_fill_in_prog_lock);
> +		write_unlock(&pkc->blk_fill_in_prog_lock);
>  	}
>  
>  	if (pkc->last_kactive_blk_num == pkc->kactive_blk_num) {
> @@ -921,10 +921,9 @@ static void prb_retire_current_block(struct tpacket_kbdq_core *pkc,
>  		 * the timer-handler already handled this case.
>  		 */
>  		if (!(status & TP_STATUS_BLK_TMO)) {
> -			while (atomic_read(&pkc->blk_fill_in_prog)) {
> -				/* Waiting for skb_copy_bits to finish... */
> -				cpu_relax();
> -			}
> +			/* Waiting for skb_copy_bits to finish... */
> +			write_lock(&pkc->blk_fill_in_prog_lock);
> +			write_unlock(&pkc->blk_fill_in_prog_lock);
>  		}
>  		prb_close_block(pkc, pbd, po, status);
>  		return;
> @@ -944,7 +943,8 @@ static int prb_queue_frozen(struct tpacket_kbdq_core *pkc)
>  static void prb_clear_blk_fill_status(struct packet_ring_buffer *rb)
>  {
>  	struct tpacket_kbdq_core *pkc  = GET_PBDQC_FROM_RB(rb);
> -	atomic_dec(&pkc->blk_fill_in_prog);
> +
> +	read_unlock(&pkc->blk_fill_in_prog_lock);
>  }
>  
>  static void prb_fill_rxhash(struct tpacket_kbdq_core *pkc,
> @@ -998,7 +998,7 @@ static void prb_fill_curr_block(char *curr,
>  	pkc->nxt_offset += TOTAL_PKT_LEN_INCL_ALIGN(len);
>  	BLOCK_LEN(pbd) += TOTAL_PKT_LEN_INCL_ALIGN(len);
>  	BLOCK_NUM_PKTS(pbd) += 1;
> -	atomic_inc(&pkc->blk_fill_in_prog);
> +	read_lock(&pkc->blk_fill_in_prog_lock);
>  	prb_run_all_ft_ops(pkc, ppd);
>  }
>  
> diff --git a/net/packet/internal.h b/net/packet/internal.h
> index 907f4cd2a718..fd41ecb7f605 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -39,7 +39,7 @@ struct tpacket_kbdq_core {
>  	char		*nxt_offset;
>  	struct sk_buff	*skb;
>  
> -	atomic_t	blk_fill_in_prog;
> +	rwlock_t	blk_fill_in_prog_lock;
>  
>  	/* Default is set to 8ms */
>  #define DEFAULT_PRB_RETIRE_TOV	(8)

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