iris: Use READ_ONCE and WRITE_ONCE for snapshots_landed

Suggested by Chris Wilson, if only to make it obvious to the human
readers that these are volatile reads.  It may also be necessary for
the compiler in a few cases.

src/gallium/drivers/iris/iris_batch.c

@@ -209,8 +209,6 @@ iris_init_batch(struct iris_batch *batch,
    iris_batch_reset(batch);
 }
 
-#define READ_ONCE(x) (*(volatile __typeof__(x) *)&(x))
-
 static struct drm_i915_gem_exec_object2 *
 find_validation_entry(struct iris_batch *batch, struct iris_bo *bo)
 {

src/gallium/drivers/iris/iris_query.c

@@ -787,7 +787,7 @@ iris_begin_query(struct pipe_context *ctx, struct pipe_query *query)
 
    q->result = 0ull;
    q->ready = false;
-   q->map->snapshots_landed = false;
+   WRITE_ONCE(q->map->snapshots_landed, false);
 
    if (q->type == PIPE_QUERY_PRIMITIVES_GENERATED && q->index == 0) {
       ice->state.prims_generated_query_active = true;
@@ -853,7 +853,7 @@ iris_check_query_no_flush(struct iris_context *ice, struct iris_query *q)
    struct iris_screen *screen = (void *) ice->ctx.screen;
    const struct gen_device_info *devinfo = &screen->devinfo;
 
-   if (!q->ready && q->map->snapshots_landed) {
+   if (!q->ready && READ_ONCE(q->map->snapshots_landed)) {
       calculate_result_on_cpu(devinfo, q);
    }
 }
@@ -874,14 +874,14 @@ iris_get_query_result(struct pipe_context *ctx,
       if (iris_batch_references(&ice->batches[q->batch_idx], bo))
          iris_batch_flush(&ice->batches[q->batch_idx]);
 
-      while (!q->map->snapshots_landed) {
+      while (!READ_ONCE(q->map->snapshots_landed)) {
          if (wait)
             iris_wait_syncpt(ctx->screen, q->syncpt, INT64_MAX);
          else
             return false;
       }
 
-      assert(q->map->snapshots_landed);
+      assert(READ_ONCE(q->map->snapshots_landed));
       calculate_result_on_cpu(devinfo, q);
    }
 
@@ -927,7 +927,7 @@ iris_get_query_result_resource(struct pipe_context *ctx,
       return;
    }
 
-   if (!q->ready && q->map->snapshots_landed) {
+   if (!q->ready && READ_ONCE(q->map->snapshots_landed)) {
       /* The final snapshots happen to have landed, so let's just compute
        * the result on the CPU now...
        */

src/gallium/drivers/iris/iris_screen.h

@@ -33,6 +33,9 @@
 
 struct iris_bo;
 
+#define READ_ONCE(x) (*(volatile __typeof__(x) *)&(x))
+#define WRITE_ONCE(x, v) *(volatile __typeof__(x) *)&(x) = (v)
+
 #define IRIS_MAX_TEXTURE_SAMPLERS 32
 #define IRIS_MAX_SOL_BUFFERS 4