diff --git a/src/system/runtime_loader/elf_tls.cpp b/src/system/runtime_loader/elf_tls.cpp
index f534cecc00..251467c70d 100644
--- a/src/system/runtime_loader/elf_tls.cpp
+++ b/src/system/runtime_loader/elf_tls.cpp
@@ -12,9 +12,14 @@
 
 #include <tls.h>
 
+#include <locks.h>
 #include <util/kernel_cpp.h>
 
 
+static const char* const kLockName = "runtime loader tls";
+static mutex sLock = MUTEX_INITIALIZER(kLockName);
+
+
 class TLSBlock {
 public:	
 	inline				TLSBlock();
@@ -280,6 +285,13 @@ DynamicThreadVector::_ResizeVector(unsigned minimumSize)
 	if (size <= oldSize)
 		return B_OK;
 
+	MutexLocker _(sLock);
+
+	// If the size has changed in the meantime, we're done.
+	oldSize = _Size();
+	if (size <= oldSize)
+		return B_OK;
+
 	void* newVector = realloc(*fVector, (size + 1) * sizeof(TLSBlock));
 	if (newVector == NULL)
 		return B_NO_MEMORY;