[Intel-gfx] [PATCH] drm/i915: Refine mmio debug flow to avoid bad unlock balance detected.

[Lee Shawn C]

Perform reboot stresss test on a kernel build with lockdebug flag enabled.
Bad unlock balanace detected would happened sometimes. Below is the
problematic scenario. If params.mmio_debug value was changed at step #4.
And it would trigger this issue. Modify code flow that decide to
enable/disable mmio debug before unclaimed_reg_debug() can avoid
this symptom.

    1. GEN6_READ_HEADER is called with params.mmio_debug = 0
    2. unclaimed_reg_debug(before = true) is called
    3. unclaimed_reg_debug return without taking a lock
       because params.mmio_debug == 0
    4. other thread modifies params.mmio_debug to 1
    5. GEN6_READ_FOOTER is called with params.mmio_debug != 0
    6. unclaimed_reg_debug tries to assert non-taken lock (first WARN)
    7. unclaimed_reg_debug tries to release non-taken lock (second WARN)

Closes:https://gitlab.freedesktop.org/drm/intel/-/issues/8749
Signed-off-by: Lee Shawn C <shawn.c.lee at intel.com>
Cc: Uma Shankar <uma.shankar at intel.com>
Cc: Matt Roper <matthew.d.roper at intel.com>
Cc: Andi Shyti <andi.shyti at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_uncore.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
index 796ebfe6c550..9d665978cc43 100644
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -1931,9 +1931,6 @@ unclaimed_reg_debug(struct intel_uncore *uncore,
 		    const bool read,
 		    const bool before)
 {
-	if (likely(!uncore->i915->params.mmio_debug) || !uncore->debug)
-		return;
-
 	/* interrupts are disabled and re-enabled around uncore->lock usage */
 	lockdep_assert_held(&uncore->lock);
 
@@ -2001,13 +1998,16 @@ __gen2_read(64)
 #define GEN6_READ_HEADER(x) \
 	u32 offset = i915_mmio_reg_offset(reg); \
 	unsigned long irqflags; \
+	const bool mmio_debug = likely(uncore->i915->params.mmio_debug) || uncore->debug; \
 	u##x val = 0; \
 	assert_rpm_wakelock_held(uncore->rpm); \
 	spin_lock_irqsave(&uncore->lock, irqflags); \
-	unclaimed_reg_debug(uncore, reg, true, true)
+	if (mmio_debug) \
+		unclaimed_reg_debug(uncore, reg, true, true)
 
 #define GEN6_READ_FOOTER \
-	unclaimed_reg_debug(uncore, reg, true, false); \
+	if (mmio_debug) \
+		unclaimed_reg_debug(uncore, reg, true, false); \
 	spin_unlock_irqrestore(&uncore->lock, irqflags); \
 	trace_i915_reg_rw(false, reg, val, sizeof(val), trace); \
 	return val
-- 
2.17.1