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

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 <[e-mail]>
Reviewed-by: Guennadi Liakhovetski <[e-mail]>
Cc: [e-mail]
Signed-off-by: Paul Mundt <[e-mail]>

drivers/tty/serial/sh-sci.c

···
 	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);
···
 	}
 
 	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) {