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

Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx> · Tue, 30 Nov 2021 13:31:08 +0100

On 11/30/21 13:25, Maarten Lankhorst wrote:
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@xxxxxxxxxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
  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?

Ah Yes :)


+
+	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. :)

Basically the problem is that within enable_signaling() which is called 
with the dma_fence lock held, we take the dma_fence lock of another 
fence. If that other fence is a dma_fence_array, or a dma_fence_chain 
which in turn tries to lock a dma_fence_array we hit a splat.

But I'll update the commit message with a typical splat.

/Thomas



~Maarten








