On 20/11/2019 12:07, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2019-11-20 11:58:27)
On 20/11/2019 09:32, Chris Wilson wrote:
In commit a79ca656b648 ("drm/i915: Push the wakeref->count deferral to
the backend"), I erroneously concluded that we last modify the engine
inside __i915_request_commit() meaning that we could enable concurrent
submission for userspace as we enqueued this request. However, this
falls into a trap with other users of the engine->kernel_context waking
up and submitting their request before the idle-switch is queued, with
the result that the kernel_context is executed out-of-sequence most
likely upsetting the GPU and certainly ourselves when we try to retire
the out-of-sequence requests.
As such we need to hold onto the effective engine->kernel_context mutex
lock (via the engine pm mutex proxy) until we have finish queuing the
request to the engine.
v2: Serialise against concurrent intel_gt_retire_requests()
v3: Describe the hairy locking scheme with intel_gt_retire_requests()
for future reference.
v4: Combine timeline->lock and engine pm release; it's hairy.
Fixes: a79ca656b648 ("drm/i915: Push the wakeref->count deferral to the backend")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_engine_pm.c | 47 +++++++++++++++++++----
1 file changed, 40 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
index 3c0f490ff2c7..1f517357a268 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
@@ -73,8 +73,25 @@ static inline void __timeline_mark_unlock(struct intel_context *ce,
#endif /* !IS_ENABLED(CONFIG_LOCKDEP) */
+static void
+__intel_timeline_enter_and_pm_release(struct intel_timeline *tl,
+ struct intel_engine_cs *engine)
+{
+ struct intel_gt_timelines *timelines = &engine->gt->timelines;
+
+ spin_lock(&timelines->lock);
+
+ if (!atomic_fetch_inc(&tl->active_count))
+ list_add_tail(&tl->link, &timelines->active_list);
Hmm with these new part it maybe matches/answers my question from
"drm/i915/gt: Close race between engine_park and
intel_gt_retire_requests". I think at least. Since it now adds a second
place timeline can enter the active_list.
But no, where does the intel_timeline_enter race come from? Can't be
userspace submission since they are blocked on wf->lock.
The race is not just with intel_timeline_enter, but with
intel_gt_retire_requests.
As soon as we are on that list, we may be retired. If we are retired
before adjusting the engine->wakeref.count, we are b0rked.
As soon as we adjust the engine->wakeref.count, another submission may
call intel_timeline_enter, and again may even retire this request. The
enter itself is perfectly fine, but we need to serialise against the
retires.
I think the two patches combined work, I am just not sure two
atomic_fetch_inc()->list_add() are needed now that you re-ordered
__i915_requests_queue and __intel_wakeref_defer_park - that's the part
which is confusing me. But it also doesn't harm...
Regards,
Tvrtko
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