serial: 8250: Fix serial8250_tx_empty() race with DMA Tx

commit 146a37e05d620cef4ad430e5d1c9c077fe6fa76f upstream.

There's a potential race before THRE/TEMT deasserts when DMA Tx is
starting up (or the next batch of continuous Tx is being submitted).
This can lead to misdetecting Tx empty condition.

It is entirely normal for THRE/TEMT to be set for some time after the
DMA Tx had been setup in serial8250_tx_dma(). As Tx side is definitely
not empty at that point, it seems incorrect for serial8250_tx_empty()
claim Tx is empty.

Fix the race by also checking in serial8250_tx_empty() whether there's
DMA Tx active.

Note: This fix only addresses in-kernel race mainly to make using
TCSADRAIN/FLUSH robust. Userspace can still cause other races but they
seem userspace concurrency control problems.

Fixes: 9ee4b83e51f74 ("serial: 8250: Add support for dmaengine")
Cc: stable@vger.kernel.org
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230317113318.31327-3-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 287153d325365a542ce14575a1acc9dcb134e5ae..1e8fe44a7099f454466e5714e5623f7ee1791507 100644 (file)
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -365,6 +365,13 @@ static inline void serial8250_do_prepare_rx_dma(struct uart_8250_port *p)
        if (dma->prepare_rx_dma)
                dma->prepare_rx_dma(p);
 }
+
+static inline bool serial8250_tx_dma_running(struct uart_8250_port *p)
+{
+       struct uart_8250_dma *dma = p->dma;
+
+       return dma && dma->tx_running;
+}
 #else
 static inline int serial8250_tx_dma(struct uart_8250_port *p)
 {
@@ -380,6 +387,11 @@ static inline int serial8250_request_dma(struct uart_8250_port *p)
        return -1;
 }
 static inline void serial8250_release_dma(struct uart_8250_port *p) { }
+
+static inline bool serial8250_tx_dma_running(struct uart_8250_port *p)
+{
+       return false;
+}
 #endif
 
 static inline int ns16550a_goto_highspeed(struct uart_8250_port *up)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 2a3bd6918c77efb251ec2459e5d3e6c3b07607b4..b82dce557c383d5a7a0e070974cce99cf35749a2 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2010,18 +2010,19 @@ static int serial8250_tx_threshold_handle_irq(struct uart_port *port)
 static unsigned int serial8250_tx_empty(struct uart_port *port)
 {
        struct uart_8250_port *up = up_to_u8250p(port);
+       unsigned int result = 0;
        unsigned long flags;
-       u16 lsr;
 
        serial8250_rpm_get(up);
 
        spin_lock_irqsave(&port->lock, flags);
-       lsr = serial_lsr_in(up);
+       if (!serial8250_tx_dma_running(up) && uart_lsr_tx_empty(serial_lsr_in(up)))
+               result = TIOCSER_TEMT;
        spin_unlock_irqrestore(&port->lock, flags);
 
        serial8250_rpm_put(up);
 
-       return uart_lsr_tx_empty(lsr) ? TIOCSER_TEMT : 0;
+       return result;
 }
 
 unsigned int serial8250_do_get_mctrl(struct uart_port *port)