dma-buf: fix dma_fence_array_signaled v4

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 <[e-mail]>
Reviewed-by: Boris Brezillon <[e-mail]>
Tested-by: Chia-I Wu <[e-mail]>
Reviewed-by: Tvrtko Ursulin <[e-mail]>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12094
Cc: <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/dma-buf/dma-fence-array.c

···
 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+	 * in the is_signalling_enabled branch might be outdated (num_pending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;
 }