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

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On 11/30/21 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);

Oh, this looks strange. Was meant to call the dma_fence_array_enable_signaling() without the lock held here.

/Thomas





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