diff --git a/libraries/wutnewlib/wut_lock.c b/libraries/wutnewlib/wut_lock.c
index 915a59a4b..b16987a7a 100644
--- a/libraries/wutnewlib/wut_lock.c
+++ b/libraries/wutnewlib/wut_lock.c
@@ -6,7 +6,7 @@
 #define MAX_LOCKS 16
 
 static OSMutex sLibcLocks[MAX_LOCKS];
-static uint32_t sLibcLockUsedMask = 0;
+static volatile uint32_t sLibcLockUsedMask = 0;
 
 static inline bool
 __wut_is_lock_valid(int *lock)
diff --git a/libraries/wutnewlib/wut_sbrk.c b/libraries/wutnewlib/wut_sbrk.c
index e9d11c8e4..2226f2584 100644
--- a/libraries/wutnewlib/wut_sbrk.c
+++ b/libraries/wutnewlib/wut_sbrk.c
@@ -14,17 +14,17 @@ void *
 __wut_sbrk_r(struct _reent *r,
              ptrdiff_t incr)
 {
-   uint32_t oldSize, newSize;
+   uint32_t newSize;
+   uint32_t oldSize = sHeapSize;
 
    do {
-      oldSize = sHeapSize;
       newSize = oldSize + incr;
 
       if (newSize > sHeapMaxSize) {
          r->_errno = ENOMEM;
          return (void *)-1;
       }
-   } while (!OSCompareAndSwapAtomic(&sHeapSize, oldSize, newSize));
+   } while (!OSCompareAndSwapAtomicEx(&sHeapSize, oldSize, newSize, &oldSize));
 
    return ((uint8_t *)sHeapBase) + oldSize;
 }