drm/xe/ct: serialise fast_lock during CT disable

The fast-path CT could be running as we enter a runtime-suspend or
potentially a GT reset, however here we only use the ct->fast_lock and
not the full ct->lock. Before disabling the CT, also serialise against
the fast_lock to ensure any in-progress work finishes before we start
nuking the CT related stuff. Once we disable ct->enabled and drop the
lock, any new work should fail gracefully, and anything that was in
progress should be finished.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_guc_ct.c

@@ -301,8 +301,10 @@ int xe_guc_ct_enable(struct xe_guc_ct *ct)
 		goto err_out;
 
 	mutex_lock(&ct->lock);
+	spin_lock_irq(&ct->fast_lock);
 	ct->g2h_outstanding = 0;
 	ct->enabled = true;
+	spin_unlock_irq(&ct->fast_lock);
 	mutex_unlock(&ct->lock);
 
 	smp_mb();
@@ -319,8 +321,10 @@ int xe_guc_ct_enable(struct xe_guc_ct *ct)
 
 void xe_guc_ct_disable(struct xe_guc_ct *ct)
 {
-	mutex_lock(&ct->lock);
-	ct->enabled = false;
+	mutex_lock(&ct->lock); /* Serialise dequeue_one_g2h() */
+	spin_lock_irq(&ct->fast_lock); /* Serialise CT fast-path */
+	ct->enabled = false; /* Finally disable CT communication */
+	spin_unlock_irq(&ct->fast_lock);
 	mutex_unlock(&ct->lock);
 
 	xa_destroy(&ct->fence_lookup);