[PATCH RESEND] serial: imx: Fix potential deadlock on sport->port.lock

[Chengfeng Ye <dg573847474@gmail.com>]

As &sport->port.lock is acquired under irq context along the following
call chain from imx_uart_rtsint(), other acquisition of the same lock
inside process context or softirq context should disable irq avoid double
lock.

<deadlock #1>

imx_uart_dma_rx_callback()
--> spin_lock(&sport->port.lock)
<interrupt>
   --> imx_uart_rtsint()
   --> spin_lock(&sport->port.lock)

This flaw was found by an experimental static analysis tool I am
developing for irq-related deadlock.

To prevent the potential deadlock, the patch uses spin_lock_irqsave()
on the &sport->port.lock inside imx_uart_dma_rx_callback() to prevent
the possible deadlock scenario.

Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
---
 drivers/tty/serial/imx.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 13cb78340709..7bb3aa19d51c 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -1165,13 +1165,14 @@ static void imx_uart_dma_rx_callback(void *data)
 	unsigned int w_bytes = 0;
 	unsigned int r_bytes;
 	unsigned int bd_size;
+	unsigned long flags;
 
 	status = dmaengine_tx_status(chan, sport->rx_cookie, &state);
 
 	if (status == DMA_ERROR) {
-		spin_lock(&sport->port.lock);
+		spin_lock_irqsave(&sport->port.lock, flags);
 		imx_uart_clear_rx_errors(sport);
-		spin_unlock(&sport->port.lock);
+		spin_unlock_irqrestore(&sport->port.lock, flags);
 		return;
 	}
 
@@ -1200,9 +1201,9 @@ static void imx_uart_dma_rx_callback(void *data)
 		r_bytes = rx_ring->head - rx_ring->tail;
 
 		/* If we received something, check for 0xff flood */
-		spin_lock(&sport->port.lock);
+		spin_lock_irqsave(&sport->port.lock, flags);
 		imx_uart_check_flood(sport, imx_uart_readl(sport, USR2));
-		spin_unlock(&sport->port.lock);
+		spin_unlock_irqrestore(&sport->port.lock, flags);
 
 		if (!(sport->port.ignore_status_mask & URXD_DUMMY_READ)) {
 
-- 
2.17.1