Re: [PATCH] dmaengine: pl330: fix descriptor allocation fail

[Vinod Koul <vinod.koul@intel.com>]

On Fri, Sep 08, 2017 at 01:00:26PM +0300, Alexander Kochetkov wrote:
> Thread A calls pl330_get_desc() to get descriptor. If DMAC descriptor
> pool is empty pl330_get_desc() allocates new descriptor using add_desc()
> and then get newly allocated descriptor using pluck_desc().
> It is possible that another concurrent thread B calls pluck_desc()
> and catch newly allocated descriptor. In that case descriptor allocation
> for thread A will fail with:
> 
> kernel: dma-pl330 20078000.dma-controller: pl330_get_desc:2469 ALERT!
> 
> The commit fix the issue by calling _add_desc() to allocate new descriptor
> to the local on stack pool and than get it from local pool. So the issue
> described will nether happen.

Tested-by please...

> 
> Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
> ---
>  drivers/dma/pl330.c |   44 +++++++++++++++++++++++++++-----------------
>  1 file changed, 27 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> index f37f497..0e7f6c9 100644
> --- a/drivers/dma/pl330.c
> +++ b/drivers/dma/pl330.c
> @@ -2417,7 +2417,8 @@ 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)

right justifed please

>  {
>  	struct dma_pl330_desc *desc;
>  	unsigned long flags;
> @@ -2427,27 +2428,33 @@ 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 int add_desc(struct pl330_dmac *pl330, gfp_t flg, int count)
> +{
> +	return _add_desc(&pl330->desc_pool, &pl330->pool_lock, flg, count);

hmmm why add a wrapper?

> +}
> +
> +static struct dma_pl330_desc *_pluck_desc(struct list_head *pool,
> +	spinlock_t *lock)

here too, it helps in readability

>  {
>  	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);
> @@ -2456,11 +2463,16 @@ 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;
>  }
>  
> +static struct dma_pl330_desc *pluck_desc(struct pl330_dmac *pl330)
> +{
> +	return _pluck_desc(&pl330->desc_pool, &pl330->pool_lock);

one more wrapper why, we dont have any logic here!

> +}
> +
>  static struct dma_pl330_desc *pl330_get_desc(struct dma_pl330_chan *pch)
>  {
>  	struct pl330_dmac *pl330 = pch->dmac;
> @@ -2472,16 +2484,14 @@ static struct dma_pl330_desc *pl330_get_desc(struct dma_pl330_chan *pch)
>  
>  	/* 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 */
> -- 
> 1.7.9.5
> 

-- 
~Vinod