On 22/11/2024 15:30, Christian König wrote:
The function silently assumed that signaling was already enabled for the
dma_fence_array. This meant that without enabling signaling first we would
never see forward progress.
Fix that by falling back to testing each individual fence when signaling
isn't enabled yet.
v2: add the comment suggested by Boris why this is done this way
v3: fix the underflow pointed out by Tvrtko
v4: atomic_read_acquire() as suggested by Tvrtko
Signed-off-by: Christian König <christian.koenig@xxxxxxx>
Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
Tested-by: Chia-I Wu <olvaffe@xxxxxxxxx>
---
drivers/dma-buf/dma-fence-array.c | 28 +++++++++++++++++++++++++++-
1 file changed, 27 insertions(+), 1 deletion(-)
diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
index 8a08ffde31e7..6657d4b30af9 100644
--- a/drivers/dma-buf/dma-fence-array.c
+++ b/drivers/dma-buf/dma-fence-array.c
@@ -103,10 +103,36 @@ static bool dma_fence_array_enable_signaling(struct dma_fence *fence)
static bool dma_fence_array_signaled(struct dma_fence *fence)
{
struct dma_fence_array *array = to_dma_fence_array(fence);
+ int num_pending;
+ unsigned int i;
- if (atomic_read(&array->num_pending) > 0)
+ /*
+ * We need to read num_pending before checking the enable_signal bit
+ * to avoid racing with the enable_signaling() implementation, which
+ * might decrement the counter, and cause a partial check.
+ * atomic_read_acquire() pairs with atomic_dec_and_test() in
+ * dma_fence_array_enable_signaling()
+ *
+ * The !--num_pending check is here to account for the any_signaled case
+ * if we race with enable_signaling(), that means the !num_pending check
+ * in the is_signalling_enabled branch might be outdated (num_pending
+ * might have been decremented), but that's fine. The user will get the
+ * right value when testing again later.
+ */
+ num_pending = atomic_read_acquire(&array->num_pending);
+ if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &array->base.flags)) {
+ if (num_pending <= 0)
+ goto signal;
return false;
+ }
+
+ for (i = 0; i < array->num_fences; ++i) {
+ if (dma_fence_is_signaled(array->fences[i]) && !--num_pending)
+ goto signal;
+ }
+ return false;
+signal:
dma_fence_array_clear_pending_error(array);
return true;
}
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
Regards,
Tvrtko
