[Intel-gfx] [PATCH 2/2] drm/i915/execlists: Skip nested spinlock for validating pending

[Chris Wilson]

Only along the submission path can we guarantee that the locked request
is indeed from a foreign engine, and so the nesting of engine/rq is
permissible. On the submission tasklet (process_csb()), we may find
ourselves competing with the normal nesting of rq/engine, invalidating
our nesting. As we only use the spinlock for debug purposes, skip the
debug if we cannot acquire the spinlock for safe validation - catching
99% of the bugs is better than causing a hard lockup.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 37ab9742abe7..b411e4ce6771 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1300,7 +1300,6 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
 	}
 
 	for (port = execlists->pending; (rq = *port); port++) {
-		unsigned long flags;
 		bool ok = true;
 
 		GEM_BUG_ON(!kref_read(&rq->fence.refcount));
@@ -1315,8 +1314,8 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
 		ce = rq->hw_context;
 
 		/* Hold tightly onto the lock to prevent concurrent retires! */
-		spin_lock_irqsave_nested(&rq->lock, flags,
-					 SINGLE_DEPTH_NESTING);
+		if (!spin_trylock(&rq->lock))
+			continue;
 
 		if (i915_request_completed(rq))
 			goto unlock;
@@ -1347,7 +1346,7 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
 		}
 
 unlock:
-		spin_unlock_irqrestore(&rq->lock, flags);
+		spin_unlock(&rq->lock);
 		if (!ok)
 			return false;
 	}
-- 
2.24.0