As DMA Rx can be completed from two places, it is possible that DMA Rx
completes before DMA completion callback had a chance to complete it.
Once the previous DMA Rx has been completed, a new one can be started
on the next UART interrupt. The following race is possible
(uart_unlock_and_check_sysrq_irqrestore() replaced with
spin_unlock_irqrestore() for simplicity/clarity):

CPU0					CPU1
					dma_rx_complete()
serial8250_handle_irq()
  spin_lock_irqsave(&port->lock)
  handle_rx_dma()
    serial8250_rx_dma_flush()
      __dma_rx_complete()
        dma->rx_running = 0
        // Complete DMA Rx
  spin_unlock_irqrestore(&port->lock)

serial8250_handle_irq()
  spin_lock_irqsave(&port->lock)
  handle_rx_dma()
    serial8250_rx_dma()
      dma->rx_running = 1
      // Setup a new DMA Rx
  spin_unlock_irqrestore(&port->lock)

					  spin_lock_irqsave(&port->lock)
					  // sees dma->rx_running = 1
					  __dma_rx_complete()
					    dma->rx_running = 0
					    // Incorrectly complete
					    // running DMA Rx

This race seems somewhat theoretical to occur for real but handle it
correctly regardless. Check what is the DMA status before complething
anything in __dma_rx_complete().

Reported-by: Gilles BULOZ <gilles.buloz@kontron.com>
Tested-by: Gilles BULOZ <gilles.buloz@kontron.com>
Fixes: 9ee4b83e51f7 ("serial: 8250: Add support for dmaengine")
Cc: stable@vger.kernel.org
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---
 drivers/tty/serial/8250/8250_dma.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 5594883a96f8..7fa66501792d 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -43,15 +43,23 @@ static void __dma_rx_complete(struct uart_8250_port *p)
 	struct uart_8250_dma	*dma = p->dma;
 	struct tty_port		*tty_port = &p->port.state->port;
 	struct dma_tx_state	state;
+	enum dma_status		dma_status;
 	int			count;
 
-	dma->rx_running = 0;
-	dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
+	/*
+	 * New DMA Rx can be started during the completion handler before it
+	 * could acquire port's lock and it might still be ongoing. Don't to
+	 * anything in such case.
+	 */
+	dma_status = dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
+	if (dma_status == DMA_IN_PROGRESS)
+		return;
 
 	count = dma->rx_size - state.residue;
 
 	tty_insert_flip_string(tty_port, dma->rx_buf, count);
 	p->port.icount.rx += count;
+	dma->rx_running = 0;
 
 	tty_flip_buffer_push(tty_port);
 }
-- 
2.30.2

On Mon, Jan 30, 2023 at 01:48:41PM +0200, Ilpo Järvinen wrote:
> As DMA Rx can be completed from two places, it is possible that DMA Rx
> completes before DMA completion callback had a chance to complete it.
> Once the previous DMA Rx has been completed, a new one can be started
> on the next UART interrupt. The following race is possible
> (uart_unlock_and_check_sysrq_irqrestore() replaced with
> spin_unlock_irqrestore() for simplicity/clarity):
> 
> CPU0					CPU1
> 					dma_rx_complete()
> serial8250_handle_irq()
>   spin_lock_irqsave(&port->lock)
>   handle_rx_dma()
>     serial8250_rx_dma_flush()
>       __dma_rx_complete()
>         dma->rx_running = 0
>         // Complete DMA Rx
>   spin_unlock_irqrestore(&port->lock)
> 
> serial8250_handle_irq()
>   spin_lock_irqsave(&port->lock)
>   handle_rx_dma()
>     serial8250_rx_dma()
>       dma->rx_running = 1
>       // Setup a new DMA Rx
>   spin_unlock_irqrestore(&port->lock)
> 
> 					  spin_lock_irqsave(&port->lock)
> 					  // sees dma->rx_running = 1
> 					  __dma_rx_complete()
> 					    dma->rx_running = 0
> 					    // Incorrectly complete
> 					    // running DMA Rx
> 
> This race seems somewhat theoretical to occur for real but handle it
> correctly regardless. Check what is the DMA status before complething
> anything in __dma_rx_complete().
> 
> Reported-by: Gilles BULOZ <gilles.buloz@kontron.com>
> Tested-by: Gilles BULOZ <gilles.buloz@kontron.com>
> Fixes: 9ee4b83e51f7 ("serial: 8250: Add support for dmaengine")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>

> ---
>  drivers/tty/serial/8250/8250_dma.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
> index 5594883a96f8..7fa66501792d 100644
> --- a/drivers/tty/serial/8250/8250_dma.c
> +++ b/drivers/tty/serial/8250/8250_dma.c
> @@ -43,15 +43,23 @@ static void __dma_rx_complete(struct uart_8250_port *p)
>  	struct uart_8250_dma	*dma = p->dma;
>  	struct tty_port		*tty_port = &p->port.state->port;
>  	struct dma_tx_state	state;
> +	enum dma_status		dma_status;
>  	int			count;
>  
> -	dma->rx_running = 0;
> -	dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
> +	/*
> +	 * New DMA Rx can be started during the completion handler before it
> +	 * could acquire port's lock and it might still be ongoing. Don't to
> +	 * anything in such case.
> +	 */
> +	dma_status = dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
> +	if (dma_status == DMA_IN_PROGRESS)
> +		return;
>  
>  	count = dma->rx_size - state.residue;
>  
>  	tty_insert_flip_string(tty_port, dma->rx_buf, count);
>  	p->port.icount.rx += count;
> +	dma->rx_running = 0;
>  
>  	tty_flip_buffer_push(tty_port);
>  }
> -- 
> 2.30.2

-- 
heikki