[RFC PATCH 1/2] dma-fence: Avoid establishing a locking order between fence classes

[Maarten Lankhorst]

On 30-11-2021 13:19, Thomas Hellström wrote:
> The locking order for taking two fence locks is implicitly defined in
> at least two ways in the code:
>
> 1) Fence containers first and other fences next, which is defined by
> the enable_signaling() callbacks of dma_fence_chain and
> dma_fence_array.
> 2) Reverse signal order, which is used by __i915_active_fence_set().
>
> Now 1) implies 2), except for the signal_on_any mode of dma_fence_array
> and 2) does not imply 1), and also 1) makes locking order between
> different containers confusing.
>
> Establish 2) and fix up the signal_on_any mode by calling
> enable_signaling() on such fences unlocked at creation.
>
> Cc: linaro-mm-sig at lists.linaro.org
> Cc: dri-devel at lists.freedesktop.org
> Cc: Christian König <christian.koenig at amd.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> ---
>  drivers/dma-buf/dma-fence-array.c | 13 +++--
>  drivers/dma-buf/dma-fence-chain.c |  3 +-
>  drivers/dma-buf/dma-fence.c       | 79 +++++++++++++++++++++----------
>  include/linux/dma-fence.h         |  3 ++
>  4 files changed, 69 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
> index 3e07f961e2f3..0322b92909fe 100644
> --- a/drivers/dma-buf/dma-fence-array.c
> +++ b/drivers/dma-buf/dma-fence-array.c
> @@ -84,8 +84,8 @@ static bool dma_fence_array_enable_signaling(struct dma_fence *fence)
>  		 * insufficient).
>  		 */
>  		dma_fence_get(&array->base);
> -		if (dma_fence_add_callback(array->fences[i], &cb[i].cb,
> -					   dma_fence_array_cb_func)) {
> +		if (dma_fence_add_callback_nested(array->fences[i], &cb[i].cb,
> +						  dma_fence_array_cb_func)) {
>  			int error = array->fences[i]->error;
>  
>  			dma_fence_array_set_pending_error(array, error);
> @@ -158,6 +158,7 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
>  {
>  	struct dma_fence_array *array;
>  	size_t size = sizeof(*array);
> +	struct dma_fence *fence;
>  
>  	/* Allocate the callback structures behind the array. */
>  	size += num_fences * sizeof(struct dma_fence_array_cb);
> @@ -165,8 +166,9 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
>  	if (!array)
>  		return NULL;
>  
> +	fence = &array->base;
>  	spin_lock_init(&array->lock);
> -	dma_fence_init(&array->base, &dma_fence_array_ops, &array->lock,
> +	dma_fence_init(fence, &dma_fence_array_ops, &array->lock,
>  		       context, seqno);
>  	init_irq_work(&array->work, irq_dma_fence_array_work);
>  
> @@ -174,7 +176,10 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
>  	atomic_set(&array->num_pending, signal_on_any ? 1 : num_fences);
>  	array->fences = fences;
>  
> -	array->base.error = PENDING_ERROR;
> +	fence->error = PENDING_ERROR;
> +
> +	if (signal_on_any)
> +		dma_fence_enable_sw_signaling(fence);
>  
>  	return array;
>  }
> diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
> index 1b4cb3e5cec9..0518e53880f6 100644
> --- a/drivers/dma-buf/dma-fence-chain.c
> +++ b/drivers/dma-buf/dma-fence-chain.c
> @@ -152,7 +152,8 @@ static bool dma_fence_chain_enable_signaling(struct dma_fence *fence)
>  		struct dma_fence *f = chain ? chain->fence : fence;
>  
>  		dma_fence_get(f);
> -		if (!dma_fence_add_callback(f, &head->cb, dma_fence_chain_cb)) {
> +		if (!dma_fence_add_callback_nested(f, &head->cb,
> +						   dma_fence_chain_cb)) {
>  			dma_fence_put(fence);
>  			return true;
>  		}
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 066400ed8841..90a3d5121746 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -610,6 +610,37 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence)
>  }
>  EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
>  
> +static int __dma_fence_add_callback(struct dma_fence *fence,
> +				    struct dma_fence_cb *cb,
> +				    dma_fence_func_t func,
> +				    int nest_level)
> +{
> +	unsigned long flags;
> +	int ret = 0;
> +
> +	if (WARN_ON(!fence || !func))
> +		return -EINVAL;
> +
> +	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
> +		INIT_LIST_HEAD(&cb->node);
> +		return -ENOENT;
> +	}
> +
> +	spin_lock_irqsave_nested(fence->lock, flags, 0);
Forgot to hook up nest_level here?
> +
> +	if (__dma_fence_enable_signaling(fence)) {
> +		cb->func = func;
> +		list_add_tail(&cb->node, &fence->cb_list);
> +	} else {
> +		INIT_LIST_HEAD(&cb->node);
> +		ret = -ENOENT;
> +	}
> +
> +	spin_unlock_irqrestore(fence->lock, flags);
> +
> +	return ret;
> +}
> +
>  /**
>   * dma_fence_add_callback - add a callback to be called when the fence
>   * is signaled
> @@ -635,33 +666,33 @@ EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
>  int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
>  			   dma_fence_func_t func)
>  {
> -	unsigned long flags;
> -	int ret = 0;
> -
> -	if (WARN_ON(!fence || !func))
> -		return -EINVAL;
> -
> -	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
> -		INIT_LIST_HEAD(&cb->node);
> -		return -ENOENT;
> -	}
> -
> -	spin_lock_irqsave(fence->lock, flags);
> -
> -	if (__dma_fence_enable_signaling(fence)) {
> -		cb->func = func;
> -		list_add_tail(&cb->node, &fence->cb_list);
> -	} else {
> -		INIT_LIST_HEAD(&cb->node);
> -		ret = -ENOENT;
> -	}
> -
> -	spin_unlock_irqrestore(fence->lock, flags);
> -
> -	return ret;
> +	return __dma_fence_add_callback(fence, cb, func, 0);
>  }
>  EXPORT_SYMBOL(dma_fence_add_callback);
>  

Other than that, I didn't investigate the nesting fails enough to say I can accurately review this. :)

~Maarten