drm/i915: Avoid unguarded reads from the request pointer

In commit 86aa7e760a67 ("drm/i915: Assert that the context-switch
completion matches our context") I added a read to the irq tasklet
handler that compared the on-chip status with that of our sw tracking,
using an unguarded read of the request pointer to get the context and
beyond. Whilst we hold a reference to the request, we do not hold
anything on the context and if we are unlucky it may be reaped from a
second thread retiring the request (since it may retire the request as
soon as the breadcrumb is complete, even before we finish processing the
context switch) as we try to read from the context pointer.

Avoid the racy read from underneath the request by storing the expected
result in the execlist_port[].

v2: Include commentary about port[].request being unprotected.

Fixes: 86aa7e760a67 ("drm/i915: Assert that the context-switch completion matches our context")
Reported-by: Mika Kuoppala <mika.kuoppala@intel.com>
Testcase: igt/gem_ctx_create
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170206170502.30944-2-chris@chris-wilson.co.uk

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 2c6d3655985e..896838ca502c 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -381,6 +381,7 @@ struct intel_engine_cs {
 	struct execlist_port {
 		struct drm_i915_gem_request *request;
 		unsigned int count;
+		GEM_BUG_ONLY_DECLARE(u32 context_id);
 	} execlist_port[2];
 	struct rb_root execlist_queue;
 	struct rb_node *execlist_first;