linux-kernel - Re: [PATCH] tty: serial: fsl_lpuart: fix race on RX DMA shutdown

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Message-ID: <cd278972-77c5-2118-0a47-327aa7406ec1@kernel.org>
Date:   Mon, 6 Mar 2023 10:43:13 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     "A. Sverdlin" <alexander.sverdlin@...mens.com>,
        linux-serial@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: serial: fsl_lpuart: fix race on RX DMA shutdown

On 06. 03. 23, 10:00, A. Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@...mens.com>
> 
>  From time to time DMA completion can come in the middle of DMA shutdown:
> 
> <process ctx>:				<IRQ>:
> lpuart32_shutdown()
>    lpuart_dma_shutdown()
>      del_timer_sync()
> 					lpuart_dma_rx_complete()
> 					  lpuart_copy_rx_to_tty()
> 					    mod_timer()
>      lpuart_dma_rx_free()
> 
> When the timer fires a bit later, sport->dma_rx_desc is NULL:
> 
> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
> pc : lpuart_copy_rx_to_tty+0xcc/0x5bc
> lr : lpuart_timer_func+0x1c/0x2c
> Call trace:
>   lpuart_copy_rx_to_tty
>   lpuart_timer_func
>   call_timer_fn
>   __run_timers.part.0
>   run_timer_softirq
>   __do_softirq
>   __irq_exit_rcu
>   irq_exit
>   handle_domain_irq
>   gic_handle_irq
>   call_on_irq_stack
>   do_interrupt_handler
>   ...
> 
> To fix this fold del_timer_sync() into lpuart_dma_rx_free() after
> dmaengine_terminate_sync() to make sure timer will not be re-started in
> lpuart_copy_rx_to_tty() <= lpuart_dma_rx_complete().
> 
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>

This should have some Fixes: tag, I believe.

> ---
>   drivers/tty/serial/fsl_lpuart.c | 11 +++--------
>   1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index e945f41b93d43..47c267ee22e04 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -1354,6 +1354,7 @@ static void lpuart_dma_rx_free(struct uart_port *port)
>   	struct dma_chan *chan = sport->dma_rx_chan;
>   
>   	dmaengine_terminate_sync(chan);

Maybe I'm missing something (I haven't looked into the code), but what 
happens if the timer ticks here? Won't the dmaengine be restarted there?

> +	del_timer_sync(&sport->lpuart_timer);
>   	dma_unmap_sg(chan->device->dev, &sport->rx_sgl, 1, DMA_FROM_DEVICE);
>   	kfree(sport->rx_ring.buf);
>   	sport->rx_ring.tail = 0;


-- 
js
suse labs