linux-kernel - Re: [PATCH 09/13] dmaengine: edma: check for echan->edesc => NULL in edma_dma

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Date:	Wed, 1 Oct 2014 15:00:53 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	<linux-serial@...r.kernel.org>
CC:	<linux-kernel@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <tony@...mide.com>,
	<balbi@...com>, <gregkh@...uxfoundation.org>,
	<vinod.koul@...el.com>
Subject: Re: [PATCH 09/13] dmaengine: edma: check for echan->edesc => NULL
 in edma_dma_pause()

On 09/29/2014 09:06 PM, Sebastian Andrzej Siewior wrote:
> I added book keeping of whether or not the 8250-dma driver has an RX
> transfer pending or not so we don't BUG here if it calls
> dmaengine_pause() on a channel which has not a pending transfer. Guess
> what, this is not enough.
> The following can be triggered with a busy RX channel and hackbench in
> background:
> - DMA transfer completes. The callback is delayed via
>   vchan_cookie_complete() into a tasklet so it das not happen asap.
> - hackbench keeps the system busy so the tasklet does not run "soon".
> - the UART collected enough data and generates an "timeout"-interrupt.
>   Since 8250-dma *thinks* the DMA-transfer is still pending it tries to
>   cancel it via invoking dmaengine_pause() first. This causes the segfault
>   because echan->edesc is NULL now that the transfer completed (however
>   the callback did not run yet).
> 
> With this patch we don't BUG in the scenario described.

Acked-by: Peter Ujfalusi <peter.ujfalusi@...com>

> 
> Cc: vinod.koul@...el.com
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  drivers/dma/edma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
> index 7b65633f495e..123f578d6dd3 100644
> --- a/drivers/dma/edma.c
> +++ b/drivers/dma/edma.c
> @@ -288,7 +288,7 @@ static int edma_slave_config(struct edma_chan *echan,
>  static int edma_dma_pause(struct edma_chan *echan)
>  {
>  	/* Pause/Resume only allowed with cyclic mode */
> -	if (!echan->edesc->cyclic)
> +	if (!echan->edesc || !echan->edesc->cyclic)
>  		return -EINVAL;
>  
>  	edma_pause(echan->ch_num);
> 


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