linux-kernel - Re: [PATCH V2] dmaengine: Fix client_count is countered one more incorrectly.

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Message-ID: <20221102180726.fuwwk2npsse56ius@cantor>
Date:   Wed, 2 Nov 2022 11:07:26 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Koba Ko <koba.ko@...onical.com>, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jie Hai <haijie1@...wei.com>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH V2] dmaengine: Fix client_count is countered one more
 incorrectly.

Hi Vinod,

Thoughts on this patch?

Maybe changing the summary to "dmaengine: Fix double increment of client_count in dma_chan_get()"
would be clearer?

On Sat, Oct 01, 2022 at 01:36:52AM +0800, Koba Ko wrote:
> If the passed client_count is 0,
> it would be incremented by balance_ref_count first
> then increment one more.
> This would cause client_count to 2.
>
> cat /sys/class/dma/dma0chan*/in_use
> 2
> 2
> 2

Would this be better?

    The first time dma_chan_get() is called for a channel the channel
    client_count is incorrectly incremented twice for public channels,
    first in balance_ref_count(), and again prior to returning. This
    results in an incorrect client count which will lead to the
    channel resources not being freed when they should be. A simple
    test of repeated module load and unload of async_tx on a Dell
    Power Edge R7425 also shows this resulting in a kref underflow
    warning.


Regards,
Jerry

>
> Fixes: d2f4f99db3e9 ("dmaengine: Rework dma_chan_get")
> Signed-off-by: Koba Ko <koba.ko@...onical.com>
> Reviewed-by: Jie Hai <haijie1@...wei.com>
> Test-by: Jie Hai <haijie1@...wei.com>
> Reviewed-by: Jerry Snitselaar <jsnitsel@...hat.com>
> Reviewed-by: Dave Jiang <dave.jiang@...el.com>
> 
> ---
> V2: Remove [3/3] on subject.
> ---
>  drivers/dma/dmaengine.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 2cfa8458b51be..78f8a9f3ad825 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -451,7 +451,8 @@ static int dma_chan_get(struct dma_chan *chan)
>  	/* The channel is already in use, update client count */
>  	if (chan->client_count) {
>  		__module_get(owner);
> -		goto out;
> +		chan->client_count++;
> +		return 0;
>  	}
>  
>  	if (!try_module_get(owner))
> @@ -470,11 +471,11 @@ static int dma_chan_get(struct dma_chan *chan)
>  			goto err_out;
>  	}
>  
> +	chan->client_count++;
> +
>  	if (!dma_has_cap(DMA_PRIVATE, chan->device->cap_mask))
>  		balance_ref_count(chan);
>  
> -out:
> -	chan->client_count++;
>  	return 0;
>  
>  err_out:
> -- 
> 2.25.1
>