Commit 632ca50f authored by John Ogness's avatar John Ogness Committed by Jakub Kicinski
Browse files

af_packet: TPACKET_V3: replace busy-wait loop 



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: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 999cf8ae

net/packet/af_packet.c

+10 −10
Original line number Diff line number Diff line
@@ -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();

		}

		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();

			}

			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);

}

net/packet/internal.h

+1 −1
Original line number Diff line number Diff line
@@ -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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