1 files changed, 17 insertions, 0 deletions
diff --git a/indra/llcommon/lockstatic.h b/indra/llcommon/lockstatic.h
index 5f08742cae..96c53c6473 100644
--- a/indra/llcommon/lockstatic.h
+++ b/indra/llcommon/lockstatic.h
@@ -46,6 +46,23 @@ protected:
 private:
     Static* getStatic()
     {
+        // Static::mMutex must be function-local static rather than class-
+        // static. Some of our consumers must function properly (therefore
+        // lock properly) even when the containing module's static variables
+        // have not yet been runtime-initialized. A mutex requires
+        // construction. A static class member might not yet have been
+        // constructed.
+        //
+        // We could store a dumb mutex_t*, notice when it's NULL and allocate a
+        // heap mutex -- but that's vulnerable to race conditions. And we can't
+        // defend the dumb pointer with another mutex.
+        //
+        // We could store a std::atomic<mutex_t*> -- but a default-constructed
+        // std::atomic<T> does not contain a valid T, even a default-constructed
+        // T! Which means std::atomic, too, requires runtime initialization.
+        //
+        // But a function-local static is guaranteed to be initialized exactly
+        // once: the first time control reaches that declaration.
         static Static sData;
         return &sData;
     }