SL-20546: Defend llrand's random generator against concurrent access

by making it thread_local.

1 files changed, 5 insertions, 13 deletions

diff --git a/indra/llcommon/llrand.cpp b/indra/llcommon/llrand.cpp
index 8206bf8e0c..e4065e23bf 100644
--- a/indra/llcommon/llrand.cpp
+++ b/indra/llcommon/llrand.cpp
@@ -28,7 +28,6 @@
 
 #include "llrand.h"
 #include "lluuid.h"
-#include "mutex.h"
 
 /**
  * Through analysis, we have decided that we want to take values which
@@ -59,16 +58,9 @@
  * to restore uniform distribution.
  */
 
-static std::mutex gRandomGeneratorMutex;
-static LLRandLagFib2281 gRandomGenerator(LLUUID::getRandomSeed());
-
-inline F64 ll_internal_random_unclamped()
-{
-	// gRandomGenerator is a stateful static object, which is therefore not
-	// inherently thread-safe. Lock it before use.
-	std::unique_lock lk(gRandomGeneratorMutex);
-	return gRandomGenerator();
-}
+// gRandomGenerator is a stateful static object, which is therefore not
+// inherently thread-safe.
+static thread_local LLRandLagFib2281 gRandomGenerator(LLUUID::getRandomSeed());
 
 // no default implementation, only specific F64 and F32 specializations
 template <typename REAL>
@@ -81,7 +73,7 @@ inline F64 ll_internal_random<F64>()
 	// CPUs (or at least multi-threaded processes) seem to
 	// occasionally give an obviously incorrect random number -- like
 	// 5^15 or something. Sooooo, clamp it as described above.
-	F64 rv{ ll_internal_random_unclamped() };
+	F64 rv{ gRandomGenerator() };
 	if(!((rv >= 0.0) && (rv < 1.0))) return fmod(rv, 1.0);
 	return rv;
 }
@@ -93,7 +85,7 @@ inline F32 ll_internal_random<F32>()
 	// Per Monty, it's important to clamp using the correct fmodf() rather
 	// than expanding to F64 for fmod() and then truncating back to F32. Prior
 	// to this change, we were getting sporadic ll_frand() == 1.0 results.
-	F32 rv{ narrow(ll_internal_random_unclamped()) };
+	F32 rv{ narrow(gRandomGenerator()) };
 	if(!((rv >= 0.0f) && (rv < 1.0f))) return fmodf(rv, 1.0f);
 	return rv;
 }