On 21/05/2020 10:44, Chris Wilson wrote:
Quoting Chris Wilson (2020-05-21 10:32:49)
Quoting Chris Wilson (2020-05-21 10:27:16)
Quoting Tvrtko Ursulin (2020-05-21 10:13:14)
On 21/05/2020 09:53, Chris Wilson wrote:
In order to be valid to dereference during the i915_fence_release, after
retiring the fence and releasing its refererences, we assume that
rq->engine can only be a real engine (that stay intact until the device
is shutdown after all fences have been flushed). However, due to a quirk
of preempt-to-busy, we may retire a request that still belongs to a
virtual engine and so eventually free it with rq->engine being invalid.
To avoid dereferencing that invalid engine, we look at the
execution_mask which if it indicates it may be executed on more than one
engine, we know it originated on a virtual engine and may still be on
one.
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1906
Fixes: 43acd6516ca9 ("drm/i915: Keep a per-engine request pool")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_request.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 526c1e9acbd5..6e357183bece 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -121,8 +121,29 @@ static void i915_fence_release(struct dma_fence *fence)
i915_sw_fence_fini(&rq->submit);
i915_sw_fence_fini(&rq->semaphore);
- /* Keep one request on each engine for reserved use under mempressure */
- if (!cmpxchg(&rq->engine->request_pool, NULL, rq))
+ /*
+ * Keep one request on each engine for reserved use under mempressure
+ *
+ * We do not hold a reference to the engine here and so have to be
+ * very careful in what rq->engine we poke. The virtual engine is
+ * referenced via the rq->context and we released that ref during
+ * i915_request_retire(), ergo we must not dereference a virtual
+ * engine here. Not that we would want to, as the only consumer of
+ * the reserved engine->request_pool is the powermanagent parking,
power management
+ * which must-not-fail, and that is only run on the physical engines.
+ *
+ * Since the request must have been executed to be have completed,
+ * we know that it will have been processed by the HW and will
+ * not be unsubmitted again, so rq->engine and rq->execution_mask
+ * at this point is stable. rq->execution_mask will be a single
+ * bit if the last and only engine it could execution on was a
+ * physical engine, if it's multiple bits then it started on and
+ * could still be on a virtual engine. Thus if the mask is not a
+ * power-of-two we assume that rq->engine may still be a virtual
+ * engien and so a dangling invalid pointer that we cannot
engine
But.. submit fence can mask out execution_mask bits and make it appear
the request was on a physical engine. What then?
Then we execute along a single engine and it is never returned to the
virtual engine (in __unwind_incomplete_requests).
* For example, consider the flow of a bonded request through a virtual
* engine. The request is created with a wide engine mask (all engines
* that we might execute on). On processing the bond, the request mask
* is reduced to one or more engines. If the request is subsequently
* bound to a single engine, it will then be constrained to only
* execute on that engine and never returned to the virtual engine
* after timeslicing away, see __unwind_incomplete_requests(). Thus we
* know that if the rq->execution_mask is a single bit, only rq->engine
rq->engine can only be a physical engine, with the exact corresponding mask.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
* can be the exact corresponding engine->mask.
-Chris
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