serial: sh-sci: fix a race of DMA submit_tx on transfer

commit 49d4bcaddca977fffdea8b0b71f6e5da96dac78e upstream.

When DMA is enabled, sh-sci transfer begins with
 uart_start()
  sci_start_tx()
    if (cookie_tx < 0) schedule_work()
Then, starts DMA when wq scheduled, -- (A)
 process_one_work()
  work_fn_rx()
   cookie_tx = desc->submit_tx()
And finishes when DMA transfer ends, -- (B)
 sci_dma_tx_complete()
  async_tx_ack()
  cookie_tx = -EINVAL
  (possible another schedule_work())

This A to B sequence is not reentrant, since controlling variables
(for example, cookie_tx above) are not queues nor lists. So, they
must be invoked as A B A B..., otherwise results in kernel crash.

To ensure the sequence, sci_start_tx() seems to test if cookie_tx < 0
(represents "not used") to call schedule_work().
But cookie_tx will not be set (to a cookie, also means "used") until
in the middle of work queue scheduled function work_fn_tx().

This gap between the test and set allows the breakage of the sequence
under the very frequently call of uart_start().
Another gap between async_tx_ack() and another schedule_work() results
in the same issue, too.

This patch introduces a new condition "cookie_tx == 0" just to mark
it is "busy" and assign it within spin-locked region to fill the gaps.

Signed-off-by: Takashi Yoshii <takashi.yoshii.zj@renesas.com>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index aff9d61..829e51a 100644 (file)
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1123,17 +1123,20 @@ static void sci_dma_tx_complete(void *arg)
        port->icount.tx += sg_dma_len(&s->sg_tx);
 
        async_tx_ack(s->desc_tx);
-       s->cookie_tx = -EINVAL;
        s->desc_tx = NULL;
 
        if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
                uart_write_wakeup(port);
 
        if (!uart_circ_empty(xmit)) {
+               s->cookie_tx = 0;
                schedule_work(&s->work_tx);
-       } else if (port->type == PORT_SCIFA || port->type == PORT_SCIFB) {
-               u16 ctrl = sci_in(port, SCSCR);
-               sci_out(port, SCSCR, ctrl & ~SCSCR_TIE);
+       } else {
+               s->cookie_tx = -EINVAL;
+               if (port->type == PORT_SCIFA || port->type == PORT_SCIFB) {
+                       u16 ctrl = sci_in(port, SCSCR);
+                       sci_out(port, SCSCR, ctrl & ~SCSCR_TIE);
+               }
        }
 
        spin_unlock_irqrestore(&port->lock, flags);
@@ -1395,8 +1398,10 @@ static void sci_start_tx(struct uart_port *port)
        }
 
        if (s->chan_tx && !uart_circ_empty(&s->port.state->xmit) &&
-           s->cookie_tx < 0)
+           s->cookie_tx < 0) {
+               s->cookie_tx = 0;
                schedule_work(&s->work_tx);
+       }
 #endif
 
        if (!s->chan_tx || port->type == PORT_SCIFA || port->type == PORT_SCIFB) {