serial: 8250_dma: Fix DMA Rx completion race

__dma_rx_complete() is called from two places:
- Through the DMA completion callback dma_rx_complete()
- From serial8250_rx_dma_flush() after IIR_RLSI or IIR_RX_TIMEOUT
The former does not hold port's lock during __dma_rx_complete() which
allows these two to race and potentially insert the same data twice.

Extend port's lock coverage in dma_rx_complete() to prevent the race
and check if the DMA Rx is still pending completion before calling
into __dma_rx_complete().

Reported-by: Gilles BULOZ <[e-mail]>
Tested-by: Gilles BULOZ <[e-mail]>
Fixes: 9ee4b83e51f7 ("serial: 8250: Add support for dmaengine")
Cc: [e-mail]
Signed-off-by: Ilpo Järvinen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_dma.c

···
 	struct uart_8250_dma *dma = p->dma;
 	unsigned long flags;
 
-	__dma_rx_complete(p);
-
 	spin_lock_irqsave(&p->port.lock, flags);
+	if (dma->rx_running)
+		__dma_rx_complete(p);
+
+	/*
+	 * Cannot be combined with the previous check because __dma_rx_complete()
+	 * changes dma->rx_running.
+	 */
 	if (!dma->rx_running && (serial_lsr_in(p) & UART_LSR_DR))
 		p->dma->rx_dma(p);
 	spin_unlock_irqrestore(&p->port.lock, flags);