[Intel-gfx] [PATCH] drm/i915: Take care of ring->last_context on ctx destroy

[Mika Kuoppala]

If the context being destroyed have been last in the ring,
the ring->last_context will be left dangling.

Later, the unpinning will happen for last_context, and as it
was already freed, we corrupt memory by doing so.

This regression introduced in
commit 6d65ba943a2d1e4292a07ca7ddb6c5138b9efa5d
Author: Nick Hoath <nicholas.hoath at intel.com>
Date:   Tue Dec 1 14:48:57 2015 +0000

    drm/i915: Extend LRC pinning to cover GPU context writeback

Fix this by clearing the ring->last_context if it is the
context being destroyed.

Cc: Nick Hoath <nicholas.hoath at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: David Gordon <david.s.gordon at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Alex Dai <yu.dai at intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index c3504a0..5c26fde 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -2432,6 +2432,9 @@ intel_lr_context_clean_ring(struct intel_context *ctx,
 		}
 	}
 
+	if (ring->last_context == ctx)
+		ring->last_context = NULL;
+
 	WARN_ON(ctx->engine[ring->id].pin_count);
 	intel_ringbuffer_free(ringbuf);
 	drm_gem_object_unreference(&ctx_obj->base);
-- 
2.5.0