drm/i915: relative_constants_mode race fix

dev_priv keeps track of the current addressing mode that gets set at
execbuffer time. Unfortunately the existing code was doing this before
acquiring struct_mutex which leaves a race with another thread also
doing an execbuffer. If that wasn't bad enough, relocate_slow drops
struct_mutex which opens a much more likely error where another thread
comes in and modifies the state while relocate_slow is being slow.

The solution here is to just defer setting this state until we
absolutely need it, and we know we'll have struct_mutex for the
remainder of our code path.

v2: Keith noticed a bug in the original patch.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index c681dc1..68e5b41 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1033,19 +1033,6 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
                        if (INTEL_INFO(dev)->gen > 5 &&
                            mode == I915_EXEC_CONSTANTS_REL_SURFACE)
                                return -EINVAL;
-
-                       ret = intel_ring_begin(ring, 4);
-                       if (ret)
-                               return ret;
-
-                       intel_ring_emit(ring, MI_NOOP);
-                       intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(1));
-                       intel_ring_emit(ring, INSTPM);
-                       intel_ring_emit(ring,
-                                       I915_EXEC_CONSTANTS_MASK << 16 | mode);
-                       intel_ring_advance(ring);
-
-                       dev_priv->relative_constants_mode = mode;
                }
                break;
        default:
@@ -1176,6 +1163,22 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
                }
        }
 
+       if (ring == &dev_priv->ring[RCS] &&
+           mode != dev_priv->relative_constants_mode) {
+               ret = intel_ring_begin(ring, 4);
+               if (ret)
+                               goto err;
+
+               intel_ring_emit(ring, MI_NOOP);
+               intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(1));
+               intel_ring_emit(ring, INSTPM);
+               intel_ring_emit(ring,
+                               I915_EXEC_CONSTANTS_MASK << 16 | mode);
+               intel_ring_advance(ring);
+
+               dev_priv->relative_constants_mode = mode;
+       }
+
        trace_i915_gem_ring_dispatch(ring, seqno);
 
        exec_start = batch_obj->gtt_offset + args->batch_start_offset;