[ 108/167] [PATCH] drm/i915: relative_constants_mode race fix

Ben Hutchings
2012-05-09T06:10:14Z

From: Ben Widawsky <ben@bwidawsk.net>

commit e2971bdab2b761683353da383c0fd5ac704d1cca upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/gpu/drm/i915/i915_gem_execbuffer.c | 29 +++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index c681dc1..68e5b41 100644
--- 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;
--
1.7.10