[PATCH 2/2] drm/i915/guc: Don't deadlock busyness stats vs reset

[John.C.Harrison at Intel.com]

From: John Harrison <John.C.Harrison at Intel.com>

The engine busyness stats has a worker function to do things like
64bit extend the 32bit hardware counters. The GuC's reset prepare
function flushes out this worker function to ensure no corruption
happens during the reset. Unforunately, the worker function has an
infinite wait for active resets to finish before doing its work. Thus
a deadlock would occur if the worker function had actually started
just as the reset starts.

Update the worker to abort if a reset is in progress rather than
waiting for it to complete. It will still acquire the reset lock in
the case where a reset was not already in progress. So the processing
is still safe from corruption, but the deadlock can no longer occur.

Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
---
 drivers/gpu/drm/i915/gt/intel_reset.c             | 15 ++++++++++++++-
 drivers/gpu/drm/i915/gt/intel_reset.h             |  1 +
 drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c |  6 ++++--
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_reset.c b/drivers/gpu/drm/i915/gt/intel_reset.c
index 3159df6cdd492..2f48c6e4420ea 100644
--- a/drivers/gpu/drm/i915/gt/intel_reset.c
+++ b/drivers/gpu/drm/i915/gt/intel_reset.c
@@ -1407,7 +1407,7 @@ void intel_gt_handle_error(struct intel_gt *gt,
 	intel_runtime_pm_put(gt->uncore->rpm, wakeref);
 }
 
-int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
+static int _intel_gt_reset_trylock(struct intel_gt *gt, int *srcu, bool retry)
 {
 	might_lock(&gt->reset.backoff_srcu);
 	might_sleep();
@@ -1416,6 +1416,9 @@ int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
 	while (test_bit(I915_RESET_BACKOFF, &gt->reset.flags)) {
 		rcu_read_unlock();
 
+		if (!retry)
+			return -EBUSY;
+
 		if (wait_event_interruptible(gt->reset.queue,
 					     !test_bit(I915_RESET_BACKOFF,
 						       &gt->reset.flags)))
@@ -1429,6 +1432,16 @@ int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
 	return 0;
 }
 
+int intel_gt_reset_trylock_noretry(struct intel_gt *gt, int *srcu)
+{
+	return _intel_gt_reset_trylock(gt, srcu, false);
+}
+
+int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
+{
+	return _intel_gt_reset_trylock(gt, srcu, true);
+}
+
 void intel_gt_reset_unlock(struct intel_gt *gt, int tag)
 __releases(&gt->reset.backoff_srcu)
 {
diff --git a/drivers/gpu/drm/i915/gt/intel_reset.h b/drivers/gpu/drm/i915/gt/intel_reset.h
index adc734e673870..7f863726eb6a2 100644
--- a/drivers/gpu/drm/i915/gt/intel_reset.h
+++ b/drivers/gpu/drm/i915/gt/intel_reset.h
@@ -38,6 +38,7 @@ int __intel_engine_reset_bh(struct intel_engine_cs *engine,
 
 void __i915_request_reset(struct i915_request *rq, bool guilty);
 
+int __must_check intel_gt_reset_trylock_noretry(struct intel_gt *gt, int *srcu);
 int __must_check intel_gt_reset_trylock(struct intel_gt *gt, int *srcu);
 void intel_gt_reset_unlock(struct intel_gt *gt, int tag);
 
diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
index 941613be3b9dd..1fa1bc7dde3df 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
@@ -1401,9 +1401,11 @@ static void guc_timestamp_ping(struct work_struct *wrk)
 
 	/*
 	 * Synchronize with gt reset to make sure the worker does not
-	 * corrupt the engine/guc stats.
+	 * corrupt the engine/guc stats. NB: can't actually block waiting
+	 * for a reset to complete as the reset requires flushing out
+	 * any running worker thread. So waiting would deadlock.
 	 */
-	ret = intel_gt_reset_trylock(gt, &srcu);
+	ret = intel_gt_reset_trylock_noretry(gt, &srcu);
 	if (ret)
 		return;
 
-- 
2.37.3