can: mcp251xfd: mcp251xfd_handle_tefif(): fix TEF vs. TX race condition

The mcp251xfd driver uses a TX FIFO for sending CAN frames and a TX Event FIFO
(TEF) for completed TX-requests.

The TEF event handling in the mcp251xfd_handle_tefif() function has a race
condition. It first increments the tx-ring's tail counter to signal that
there's room in the TX and TEF FIFO, then it increments the TEF FIFO in
hardware.

A running mcp251xfd_start_xmit() on a different CPU might not stop the txqueue
(as the tx-ring still shows free space). The next mcp251xfd_start_xmit() will
push a message into the chip and the TX complete event might overflow the TEF
FIFO.

This patch changes the order to fix the problem.

Fixes: 68c0c1c7 ("can: mcp251xfd: tef-path: reduce number of SPI core requests to set UINC bit")
Link: https://lore.kernel.org/r/20210105214138.3150886-2-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c

@@ -1368,13 +1368,10 @@ static int mcp251xfd_handle_tefif(struct mcp251xfd_priv *priv)
 		struct mcp251xfd_tx_ring *tx_ring = priv->tx;
 		struct spi_transfer *last_xfer;
 
-		tx_ring->tail += len;
-
 		/* Increment the TEF FIFO tail pointer 'len' times in
 		 * a single SPI message.
-		 */
-
-		/* Note:
+		 *
+		 * Note:
 		 *
 		 * "cs_change == 1" on the last transfer results in an
 		 * active chip select after the complete SPI
@@ -1391,6 +1388,8 @@ static int mcp251xfd_handle_tefif(struct mcp251xfd_priv *priv)
 		if (err)
 			return err;
 
+		tx_ring->tail += len;
+
 		err = mcp251xfd_check_tef_tail(priv);
 		if (err)
 			return err;