Subject: [PATCH 7/8] drm/i915/guc: Consider also RCU depth in busy loop.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Tue, 3 Oct 2023 21:37:21 +0200

intel_guc_send_busy_loop() looks at in_atomic() and irqs_disabled() to
decide if it should busy-spin while waiting or if it may sleep.
Both checks will report false on PREEMPT_RT if sleeping spinlocks are
acquired leading to RCU splats while the function sleeps.

Check also if RCU has been disabled.

Reported-by: "John B. Wyatt IV" <[email-masked]>
Reviewed-by: Rodrigo Vivi <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 drivers/gpu/drm/i915/gt/uc/intel_guc.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/gpu/drm/i915/gt/uc/intel_guc.h
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc.h
@@ -362,7 +362,7 @@ static inline int intel_guc_send_busy_lo
 {
 	int err;
 	unsigned int sleep_period_ms = 1;
-	bool not_atomic = !in_atomic() && !irqs_disabled();
+	bool not_atomic = !in_atomic() && !irqs_disabled() && !rcu_preempt_depth();
 
 	/*
 	 * FIXME: Have caller pass in if we are in an atomic context to avoid