Linux Kernels

tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock

This change “tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock” (commit 54a44d5) in Linux kernel is authored by Nikita Yushchenko <nikita.yoush [at] cogentembedded.com> on Sun Dec 4 18:49:28 2016 +0300.

Description of "tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock"

The change “tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock” introduces changes as follows.

tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock

Problem found via lockdep:

- lpuart_set_termios() calls del_timer_sync(&sport->lpuart_timer) while
  holding sport->port.lock

- sport->lpuart_timer routine is lpuart_timer_func() that calls
  lpuart_copy_rx_to_tty() that acquires same lock.

To fix, move Rx DMA stopping out of lock, as it already is in other places
in the same file.

While at it, also make Rx DMA start/stop code to look the same is in
other places in the same file.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Tested-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Linux kernel releases containing commit 54a44d5

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock"

There are 35 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/tty/serial/fsl_lpuart.c | 35 ++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 18 deletions(-)
 
diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index a1c6519837a4..f02934ffb329 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1407,6 +1407,18 @@ lpuart_set_termios(struct uart_port *port, struct ktermios *termios,
 	/* ask the core to calculate the divisor */
 	baud = uart_get_baud_rate(port, termios, old, 50, port->uartclk / 16);
 
+	/*
+	 * Need to update the Ring buffer length according to the selected
+	 * baud rate and restart Rx DMA path.
+	 *
+	 * Since timer function acqures sport->port.lock, need to stop before
+	 * acquring same lock because otherwise del_timer_sync() can deadlock.
+	 */
+	if (old && sport->lpuart_dma_rx_use) {
+		del_timer_sync(&sport->lpuart_timer);
+		lpuart_dma_rx_free(&sport->port);
+	}
+
 	spin_lock_irqsave(&sport->port.lock, flags);
 
 	sport->port.read_status_mask = 0;
@@ -1456,22 +1468,11 @@ lpuart_set_termios(struct uart_port *port, struct ktermios *termios,
 	/* restore control register */
 	writeb(old_cr2, sport->port.membase + UARTCR2);
 
-	/*
-	 * If new baud rate is set, we will also need to update the Ring buffer
-	 * length according to the selected baud rate and restart Rx DMA path.
-	 */
-	if (old) {
-		if (sport->lpuart_dma_rx_use) {
-			del_timer_sync(&sport->lpuart_timer);
-			lpuart_dma_rx_free(&sport->port);
-		}
-
-		if (sport->dma_rx_chan && !lpuart_start_rx_dma(sport)) {
-			sport->lpuart_dma_rx_use = true;
+	if (old && sport->lpuart_dma_rx_use) {
+		if (!lpuart_start_rx_dma(sport))
 			rx_dma_timer_init(sport);
-		} else {
+		else
 			sport->lpuart_dma_rx_use = false;
-		}
 	}
 
 	spin_unlock_irqrestore(&sport->port.lock, flags);
@@ -2131,12 +2132,10 @@ static int lpuart_resume(struct device *dev)
 
 	if (sport->lpuart_dma_rx_use) {
 		if (sport->port.irq_wake) {
-			if (!lpuart_start_rx_dma(sport)) {
-				sport->lpuart_dma_rx_use = true;
+			if (!lpuart_start_rx_dma(sport))
 				rx_dma_timer_init(sport);
-			} else {
+			else
 				sport->lpuart_dma_rx_use = false;
-			}
 		}
 	}
 

The commit for this change in Linux stable tree is 54a44d5 (patch).

Last modified: 2020/01/11 09:44