drm/i915: Lock out execlist tasklet while peeking inside for busy-stats

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 it, 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@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115092041.13509-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
index d790bdc..b221610 100644 (file)
--- 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)