[Intel-xe] [PATCH] drm/xe: Fix locking in CT fast path

[Matthew Brost]

We can't sleep in the CT fast but need to ensure we can access VRAM. Use
a trylock + reference counter check to ensure safe access to VRAM, if
either check fails, fall back to slow path.

VLK-45296

Signed-off-by: Matthew Brost <matthew.brost at intel.com>
---
 drivers/gpu/drm/xe/xe_device.h |  9 ++++++++-
 drivers/gpu/drm/xe/xe_guc_ct.c | 11 ++++++++++-
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_device.h b/drivers/gpu/drm/xe/xe_device.h
index 25c5087f5aad..0cc4f52098a1 100644
--- a/drivers/gpu/drm/xe/xe_device.h
+++ b/drivers/gpu/drm/xe/xe_device.h
@@ -95,12 +95,19 @@ static inline void xe_device_assert_mem_access(struct xe_device *xe)
 	XE_WARN_ON(!xe->mem_access.ref);
 }
 
+static inline bool __xe_device_mem_access_ongoing(struct xe_device *xe)
+{
+	lockdep_assert_held(&xe->mem_access.lock);
+
+	return xe->mem_access.ref;
+}
+
 static inline bool xe_device_mem_access_ongoing(struct xe_device *xe)
 {
 	bool ret;
 
 	mutex_lock(&xe->mem_access.lock);
-	ret = xe->mem_access.ref;
+	ret = __xe_device_mem_access_ongoing(xe);
 	mutex_unlock(&xe->mem_access.lock);
 
 	return ret;
diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
index e5ed9022a0a2..bba0ef21c9e5 100644
--- a/drivers/gpu/drm/xe/xe_guc_ct.c
+++ b/drivers/gpu/drm/xe/xe_guc_ct.c
@@ -1030,9 +1030,15 @@ void xe_guc_ct_fast_path(struct xe_guc_ct *ct)
 	struct xe_device *xe = ct_to_xe(ct);
 	int len;
 
-	if (!xe_device_in_fault_mode(xe) || !xe_device_mem_access_ongoing(xe))
+	if (!xe_device_in_fault_mode(xe))
 		return;
 
+	if (!mutex_trylock(&xe->mem_access.lock))
+		return;
+
+	if (!__xe_device_mem_access_ongoing(xe))
+		goto unlock;
+
 	spin_lock(&ct->fast_lock);
 	do {
 		len = g2h_read(ct, ct->fast_msg, true);
@@ -1040,6 +1046,9 @@ void xe_guc_ct_fast_path(struct xe_guc_ct *ct)
 			g2h_fast_path(ct, ct->fast_msg, len);
 	} while (len > 0);
 	spin_unlock(&ct->fast_lock);
+
+unlock:
+	mutex_unlock(&xe->mem_access.lock);
 }
 
 /* Returns less than zero on error, 0 on done, 1 on more available */
-- 
2.34.1