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: John Ogness <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

net/packet/af_packet.c

···
 						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);
···
 	 *
 	 */
 	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) {
···
 		 * 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;
···
 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,
···
 	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

···
 	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)