linux-kernel - Re: [PATCH] drivers/dma/dma-jz4780: Fix race condition between probe and irq handler

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Date:   Thu, 20 Aug 2020 13:59:23 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     madhuparnabhowmik10@...il.com
Cc:     dan.j.williams@...el.com, vkoul@...nel.org,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        andrianov@...ras.ru, ldv-project@...uxtesting.org
Subject: Re: [PATCH] drivers/dma/dma-jz4780: Fix race condition between probe
 and irq handler

Hi,

Le dim. 16 août 2020 à 12:52, madhuparnabhowmik10@...il.com a écrit :
> From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
> 
> In probe IRQ is requested before zchan->id is initialized which can be
> read in the irq handler. Hence, shift request irq and enable clock 
> after
> other initializations complete. Here, enable clock part is not part of
> the race, it is just shifted down after request_irq to keep the error
> path same as before.
> 
> Found by Linux Driver Verification project (linuxtesting.org).
> 
> Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>

I don't think there is a race at all, the interrupt handler won't be 
called before the DMA is registered.

More importantly, this patch will break things, as there are now 
register writes in the probe before the clock is enabled.

Cheers,
-Paul

> ---
>  drivers/dma/dma-jz4780.c | 44 
> ++++++++++++++++++++--------------------
>  1 file changed, 22 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/dma/dma-jz4780.c b/drivers/dma/dma-jz4780.c
> index 448f663da89c..5cbc8c3bd6c7 100644
> --- a/drivers/dma/dma-jz4780.c
> +++ b/drivers/dma/dma-jz4780.c
> @@ -879,28 +879,6 @@ static int jz4780_dma_probe(struct 
> platform_device *pdev)
>  		return -EINVAL;
>  	}
> 
> -	ret = platform_get_irq(pdev, 0);
> -	if (ret < 0)
> -		return ret;
> -
> -	jzdma->irq = ret;
> -
> -	ret = request_irq(jzdma->irq, jz4780_dma_irq_handler, 0, 
> dev_name(dev),
> -			  jzdma);
> -	if (ret) {
> -		dev_err(dev, "failed to request IRQ %u!\n", jzdma->irq);
> -		return ret;
> -	}
> -
> -	jzdma->clk = devm_clk_get(dev, NULL);
> -	if (IS_ERR(jzdma->clk)) {
> -		dev_err(dev, "failed to get clock\n");
> -		ret = PTR_ERR(jzdma->clk);
> -		goto err_free_irq;
> -	}
> -
> -	clk_prepare_enable(jzdma->clk);
> -
>  	/* Property is optional, if it doesn't exist the value will remain 
> 0. */
>  	of_property_read_u32_index(dev->of_node, 
> "ingenic,reserved-channels",
>  				   0, &jzdma->chan_reserved);
> @@ -949,6 +927,28 @@ static int jz4780_dma_probe(struct 
> platform_device *pdev)
>  		jzchan->vchan.desc_free = jz4780_dma_desc_free;
>  	}
> 
> +	ret = platform_get_irq(pdev, 0);
> +	if (ret < 0)
> +		return ret;
> +
> +	jzdma->irq = ret;
> +
> +	ret = request_irq(jzdma->irq, jz4780_dma_irq_handler, 0, 
> dev_name(dev),
> +			  jzdma);
> +	if (ret) {
> +		dev_err(dev, "failed to request IRQ %u!\n", jzdma->irq);
> +		return ret;
> +	}
> +
> +	jzdma->clk = devm_clk_get(dev, NULL);
> +	if (IS_ERR(jzdma->clk)) {
> +		dev_err(dev, "failed to get clock\n");
> +		ret = PTR_ERR(jzdma->clk);
> +		goto err_free_irq;
> +	}
> +
> +	clk_prepare_enable(jzdma->clk);
> +
>  	ret = dmaenginem_async_device_register(dd);
>  	if (ret) {
>  		dev_err(dev, "failed to register device\n");
> --
> 2.17.1
>