Re: [PATCH v2 1/2] dmaengine: pl330: fix descriptor allocation fail

[Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>]

Hi Alexander,

It took me a while to test this patch, I've been busy with other stuff...

On 2017-10-04 13:37, Alexander Kochetkov wrote:
> If two concurrent threads call pl330_get_desc() when DMAC descriptor
> pool is empty it is possible that allocation for one of threads will fail
> with message:
>
> kernel: dma-pl330 20078000.dma-controller: pl330_get_desc:2469 ALERT!
>
> Here how that can happen. Thread A calls pl330_get_desc() to get
> descriptor. If DMAC descriptor pool is empty pl330_get_desc() allocates
> new descriptor on shared pool using add_desc() and then get newly
> allocated descriptor using pluck_desc(). At the same time thread B calls
> pluck_desc() and take newly allocated descriptor. In that case descriptor
> allocation for thread A will fail.
>
> Using on-stack pool for new descriptor allow avoid the issue described.
> The patch modify pl330_get_desc() to use on-stack pool for allocation
> new descriptors.
>
> Signed-off-by: Alexander Kochetkov <al.kochet@xxxxxxxxx>

Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

> ---
>   drivers/dma/pl330.c |   39 ++++++++++++++++++++-------------------
>   1 file changed, 20 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> index b19ee04..deec4a4 100644
> --- a/drivers/dma/pl330.c
> +++ b/drivers/dma/pl330.c
> @@ -2390,7 +2390,8 @@ static inline void _init_desc(struct dma_pl330_desc *desc)
>   }
>   
>   /* Returns the number of descriptors added to the DMAC pool */
> -static int add_desc(struct pl330_dmac *pl330, gfp_t flg, int count)
> +static int add_desc(struct list_head *pool, spinlock_t *lock,
> +		    gfp_t flg, int count)
>   {
>   	struct dma_pl330_desc *desc;
>   	unsigned long flags;
> @@ -2400,27 +2401,28 @@ static int add_desc(struct pl330_dmac *pl330, gfp_t flg, int count)
>   	if (!desc)
>   		return 0;
>   
> -	spin_lock_irqsave(&pl330->pool_lock, flags);
> +	spin_lock_irqsave(lock, flags);
>   
>   	for (i = 0; i < count; i++) {
>   		_init_desc(&desc[i]);
> -		list_add_tail(&desc[i].node, &pl330->desc_pool);
> +		list_add_tail(&desc[i].node, pool);
>   	}
>   
> -	spin_unlock_irqrestore(&pl330->pool_lock, flags);
> +	spin_unlock_irqrestore(lock, flags);
>   
>   	return count;
>   }
>   
> -static struct dma_pl330_desc *pluck_desc(struct pl330_dmac *pl330)
> +static struct dma_pl330_desc *pluck_desc(struct list_head *pool,
> +					 spinlock_t *lock)
>   {
>   	struct dma_pl330_desc *desc = NULL;
>   	unsigned long flags;
>   
> -	spin_lock_irqsave(&pl330->pool_lock, flags);
> +	spin_lock_irqsave(lock, flags);
>   
> -	if (!list_empty(&pl330->desc_pool)) {
> -		desc = list_entry(pl330->desc_pool.next,
> +	if (!list_empty(pool)) {
> +		desc = list_entry(pool->next,
>   				struct dma_pl330_desc, node);
>   
>   		list_del_init(&desc->node);
> @@ -2429,7 +2431,7 @@ static struct dma_pl330_desc *pluck_desc(struct pl330_dmac *pl330)
>   		desc->txd.callback = NULL;
>   	}
>   
> -	spin_unlock_irqrestore(&pl330->pool_lock, flags);
> +	spin_unlock_irqrestore(lock, flags);
>   
>   	return desc;
>   }
> @@ -2441,20 +2443,18 @@ static struct dma_pl330_desc *pl330_get_desc(struct dma_pl330_chan *pch)
>   	struct dma_pl330_desc *desc;
>   
>   	/* Pluck one desc from the pool of DMAC */
> -	desc = pluck_desc(pl330);
> +	desc = pluck_desc(&pl330->desc_pool, &pl330->pool_lock);
>   
>   	/* If the DMAC pool is empty, alloc new */
>   	if (!desc) {
> -		if (!add_desc(pl330, GFP_ATOMIC, 1))
> -			return NULL;
> +		DEFINE_SPINLOCK(lock);
> +		LIST_HEAD(pool);
>   
> -		/* Try again */
> -		desc = pluck_desc(pl330);
> -		if (!desc) {
> -			dev_err(pch->dmac->ddma.dev,
> -				"%s:%d ALERT!\n", __func__, __LINE__);
> +		if (!add_desc(&pool, &lock, GFP_ATOMIC, 1))
>   			return NULL;
> -		}
> +
> +		desc = pluck_desc(&pool, &lock);
> +		WARN_ON(!desc || !list_empty(&pool));
>   	}
>   
>   	/* Initialize the descriptor */
> @@ -2868,7 +2868,8 @@ static int __maybe_unused pl330_resume(struct device *dev)
>   	spin_lock_init(&pl330->pool_lock);
>   
>   	/* Create a descriptor pool of default size */
> -	if (!add_desc(pl330, GFP_KERNEL, NR_DEFAULT_DESC))
> +	if (!add_desc(&pl330->desc_pool, &pl330->pool_lock,
> +		      GFP_KERNEL, NR_DEFAULT_DESC))
>   		dev_warn(&adev->dev, "unable to allocate desc\n");
>   
>   	INIT_LIST_HEAD(&pd->channels);

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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