can: statistics: use atomic access in hot path

In can_send() and can_receive() CAN messages and CAN filter matches are
counted to be visible in the CAN procfs files.

KCSAN detected a data race within can_send() when two CAN frames have
been generated by a timer event writing to the same CAN netdevice at the
same time. Use atomic operations to access the statistics in the hot path
to fix the KCSAN complaint.

Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Oliver Hartkopp <[e-mail]>
Reviewed-by: Vincent Mailhol <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

net/can/af_can.c

···
 		netif_rx(newskb);
 
 	/* update statistics */
-	pkg_stats->tx_frames++;
-	pkg_stats->tx_frames_delta++;
+	atomic_long_inc(&pkg_stats->tx_frames);
+	atomic_long_inc(&pkg_stats->tx_frames_delta);
 
 	return 0;
 
···
 	int matches;
 
 	/* update statistics */
-	pkg_stats->rx_frames++;
-	pkg_stats->rx_frames_delta++;
+	atomic_long_inc(&pkg_stats->rx_frames);
+	atomic_long_inc(&pkg_stats->rx_frames_delta);
 
 	/* create non-zero unique skb identifier together with *skb */
 	while (!(can_skb_prv(skb)->skbcnt))
···
 	consume_skb(skb);
 
 	if (matches > 0) {
-		pkg_stats->matches++;
-		pkg_stats->matches_delta++;
+		atomic_long_inc(&pkg_stats->matches);
+		atomic_long_inc(&pkg_stats->matches_delta);
 	}
 }
 

net/can/af_can.h

···
 struct can_pkg_stats {
 	unsigned long jiffies_init;
 
-	unsigned long rx_frames;
-	unsigned long tx_frames;
-	unsigned long matches;
+	atomic_long_t rx_frames;
+	atomic_long_t tx_frames;
+	atomic_long_t matches;
 
 	unsigned long total_rx_rate;
 	unsigned long total_tx_rate;
···
 	unsigned long max_tx_rate;
 	unsigned long max_rx_match_ratio;
 
-	unsigned long rx_frames_delta;
-	unsigned long tx_frames_delta;
-	unsigned long matches_delta;
+	atomic_long_t rx_frames_delta;
+	atomic_long_t tx_frames_delta;
+	atomic_long_t matches_delta;
 };
 
 /* persistent statistics */

net/can/proc.c

···
 	struct can_pkg_stats *pkg_stats = net->can.pkg_stats;
 	unsigned long j = jiffies; /* snapshot */
 
+	long rx_frames = atomic_long_read(&pkg_stats->rx_frames);
+	long tx_frames = atomic_long_read(&pkg_stats->tx_frames);
+	long matches = atomic_long_read(&pkg_stats->matches);
+	long rx_frames_delta = atomic_long_read(&pkg_stats->rx_frames_delta);
+	long tx_frames_delta = atomic_long_read(&pkg_stats->tx_frames_delta);
+	long matches_delta = atomic_long_read(&pkg_stats->matches_delta);
+
 	/* restart counting in timer context on user request */
 	if (user_reset)
 		can_init_stats(net);
···
 		can_init_stats(net);
 
 	/* prevent overflow in calc_rate() */
-	if (pkg_stats->rx_frames > (ULONG_MAX / HZ))
+	if (rx_frames > (LONG_MAX / HZ))
 		can_init_stats(net);
 
 	/* prevent overflow in calc_rate() */
-	if (pkg_stats->tx_frames > (ULONG_MAX / HZ))
+	if (tx_frames > (LONG_MAX / HZ))
 		can_init_stats(net);
 
 	/* matches overflow - very improbable */
-	if (pkg_stats->matches > (ULONG_MAX / 100))
+	if (matches > (LONG_MAX / 100))
 		can_init_stats(net);
 
 	/* calc total values */
-	if (pkg_stats->rx_frames)
-		pkg_stats->total_rx_match_ratio = (pkg_stats->matches * 100) /
-			pkg_stats->rx_frames;
+	if (rx_frames)
+		pkg_stats->total_rx_match_ratio = (matches * 100) / rx_frames;
 
 	pkg_stats->total_tx_rate = calc_rate(pkg_stats->jiffies_init, j,
-					    pkg_stats->tx_frames);
+					    tx_frames);
 	pkg_stats->total_rx_rate = calc_rate(pkg_stats->jiffies_init, j,
-					    pkg_stats->rx_frames);
+					    rx_frames);
 
 	/* calc current values */
-	if (pkg_stats->rx_frames_delta)
+	if (rx_frames_delta)
 		pkg_stats->current_rx_match_ratio =
-			(pkg_stats->matches_delta * 100) /
-			pkg_stats->rx_frames_delta;
+			(matches_delta * 100) /	rx_frames_delta;
 
-	pkg_stats->current_tx_rate = calc_rate(0, HZ, pkg_stats->tx_frames_delta);
-	pkg_stats->current_rx_rate = calc_rate(0, HZ, pkg_stats->rx_frames_delta);
+	pkg_stats->current_tx_rate = calc_rate(0, HZ, tx_frames_delta);
+	pkg_stats->current_rx_rate = calc_rate(0, HZ, rx_frames_delta);
 
 	/* check / update maximum values */
 	if (pkg_stats->max_tx_rate < pkg_stats->current_tx_rate)
···
 		pkg_stats->max_rx_match_ratio = pkg_stats->current_rx_match_ratio;
 
 	/* clear values for 'current rate' calculation */
-	pkg_stats->tx_frames_delta = 0;
-	pkg_stats->rx_frames_delta = 0;
-	pkg_stats->matches_delta   = 0;
+	atomic_long_set(&pkg_stats->tx_frames_delta, 0);
+	atomic_long_set(&pkg_stats->rx_frames_delta, 0);
+	atomic_long_set(&pkg_stats->matches_delta, 0);
 
 	/* restart timer (one second) */
 	mod_timer(&net->can.stattimer, round_jiffies(jiffies + HZ));
···
 	struct can_rcv_lists_stats *rcv_lists_stats = net->can.rcv_lists_stats;
 
 	seq_putc(m, '\n');
-	seq_printf(m, " %8ld transmitted frames (TXF)\n", pkg_stats->tx_frames);
-	seq_printf(m, " %8ld received frames (RXF)\n", pkg_stats->rx_frames);
-	seq_printf(m, " %8ld matched frames (RXMF)\n", pkg_stats->matches);
+	seq_printf(m, " %8ld transmitted frames (TXF)\n",
+		   atomic_long_read(&pkg_stats->tx_frames));
+	seq_printf(m, " %8ld received frames (RXF)\n",
+		   atomic_long_read(&pkg_stats->rx_frames));
+	seq_printf(m, " %8ld matched frames (RXMF)\n",
+		   atomic_long_read(&pkg_stats->matches));
 
 	seq_putc(m, '\n');