[Intel-gfx] [PATCH] drm/i915: Lock out execlist tasklet while peeking inside for busy-stats

[Chris Wilson]

In order to prevent a race condition where we may end up overaccounting
the active state and leaving the busy-stats believing the GPU is 100%
busy, lock out the tasklet while we reconstruct the busy state. There is
no direct spinlock guard for the execlists->port[], so we need to
utilise tasklet_disable() as a synchronous barrier to prevent the only
writer to execlists->port[] from running at the same time as the enable.

Fixes: 4900727d35bb ("drm/i915/pmu: Reconstruct active state on starting busy-stats")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_engine_cs.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
index d790bdc227ff..b221610f2365 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -1943,16 +1943,22 @@ intel_engine_lookup_user(struct drm_i915_private *i915, u8 class, u8 instance)
  */
 int intel_enable_engine_stats(struct intel_engine_cs *engine)
 {
+	struct intel_engine_execlists *execlists = &engine->execlists;
 	unsigned long flags;
+	int err = 0;
 
 	if (!intel_engine_supports_stats(engine))
 		return -ENODEV;
 
+	tasklet_disable(&execlists->tasklet);
 	spin_lock_irqsave(&engine->stats.lock, flags);
-	if (engine->stats.enabled == ~0)
-		goto busy;
+
+	if (unlikely(engine->stats.enabled == ~0)) {
+		err = -EBUSY;
+		goto unlock;
+	}
+
 	if (engine->stats.enabled++ == 0) {
-		struct intel_engine_execlists *execlists = &engine->execlists;
 		const struct execlist_port *port = execlists->port;
 		unsigned int num_ports = execlists_num_ports(execlists);
 
@@ -1967,14 +1973,12 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine)
 		if (engine->stats.active)
 			engine->stats.start = engine->stats.enabled_at;
 	}
-	spin_unlock_irqrestore(&engine->stats.lock, flags);
 
-	return 0;
-
-busy:
+unlock:
 	spin_unlock_irqrestore(&engine->stats.lock, flags);
+	tasklet_enable(&execlists->tasklet);
 
-	return -EBUSY;
+	return err;
 }
 
 static ktime_t __intel_engine_get_busy_time(struct intel_engine_cs *engine)
-- 
2.15.1