[PATCH v3] dma-fence: Propagate errors to dma-fence-array container

[Chris Wilson]

When one of the array of fences is signaled, propagate its errors to the
parent fence-array (keeping the first error to be raised).

v2: Opencode cmpxchg_local to avoid compiler freakout.
v3: Be careful not to flag an error if we race against signal-on-any

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
Cc: Gustavo Padovan <gustavo at padovan.org>
Cc: Christian König <christian.koenig at amd.com>
---
 drivers/dma-buf/dma-fence-array.c | 25 +++++++++++++++++++++++++
 include/linux/dma-fence-array.h   |  2 ++
 2 files changed, 27 insertions(+)

diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
index 12c6f64c0bc2..e806c5fe9ec9 100644
--- a/drivers/dma-buf/dma-fence-array.c
+++ b/drivers/dma-buf/dma-fence-array.c
@@ -23,10 +23,19 @@ static const char *dma_fence_array_get_timeline_name(struct dma_fence *fence)
 	return "unbound";
 }
 
+static void dma_fence_array_set_error_once(struct dma_fence_array *array,
+					   int error)
+{
+	if (!array->base.error && error)
+		dma_fence_set_error(&array->base, error);
+}
+
 static void irq_dma_fence_array_work(struct irq_work *wrk)
 {
 	struct dma_fence_array *array = container_of(wrk, typeof(*array), work);
 
+	dma_fence_array_set_error_once(array, READ_ONCE(array->pending_error));
+
 	dma_fence_signal(&array->base);
 	dma_fence_put(&array->base);
 }
@@ -38,6 +47,19 @@ static void dma_fence_array_cb_func(struct dma_fence *f,
 		container_of(cb, struct dma_fence_array_cb, cb);
 	struct dma_fence_array *array = array_cb->array;
 
+	/*
+	 * Propagate the first error reported by any of our fences, but only
+	 * before we ourselves are signaled.
+	 *
+	 * Note that this may race with multiple fences completing
+	 * simultaneously in error, but only one error will be kept, not
+	 * necessarily the first. So long as we propagate an error if any
+	 * fences were in error before we are signaled we should be telling
+	 * an acceptable truth.
+	 */
+	if (f->error && !array->pending_error)
+		WRITE_ONCE(array->pending_error, f->error);
+
 	if (atomic_dec_and_test(&array->num_pending))
 		irq_work_queue(&array->work);
 	else
@@ -63,6 +85,8 @@ static bool dma_fence_array_enable_signaling(struct dma_fence *fence)
 		dma_fence_get(&array->base);
 		if (dma_fence_add_callback(array->fences[i], &cb[i].cb,
 					   dma_fence_array_cb_func)) {
+			dma_fence_array_set_error_once(array,
+						       array->fences[i]->error);
 			dma_fence_put(&array->base);
 			if (atomic_dec_and_test(&array->num_pending))
 				return false;
@@ -141,6 +165,7 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
 	array->num_fences = num_fences;
 	atomic_set(&array->num_pending, signal_on_any ? 1 : num_fences);
 	array->fences = fences;
+	array->pending_error = 0;
 
 	return array;
 }
diff --git a/include/linux/dma-fence-array.h b/include/linux/dma-fence-array.h
index 303dd712220f..faaf70c524ae 100644
--- a/include/linux/dma-fence-array.h
+++ b/include/linux/dma-fence-array.h
@@ -42,6 +42,8 @@ struct dma_fence_array {
 	atomic_t num_pending;
 	struct dma_fence **fences;
 
+	int pending_error;
+
 	struct irq_work work;
 };
 
-- 
2.23.0.rc1