diff --git a/redis.c b/redis.c
index 7c87178d69a0d466727cd95f0bddcf713ac1bd79..47c93f732474fdb74bbdae6461dc0b5a7f59a141 100644
--- a/redis.c
+++ b/redis.c
@@ -6999,6 +6999,9 @@ static void vmInit(void) {
     int pipefds[2];
     size_t stacksize;
 
+    if (server.vm_max_threads != 0)
+        zmalloc_enable_thread_safeness(); /* we need thread safe zmalloc() */
+
     server.vm_fp = fopen("/tmp/redisvm","w+b");
     if (server.vm_fp == NULL) {
         redisLog(REDIS_WARNING,"Impossible to open the swap file. Exiting.");
diff --git a/zmalloc.c b/zmalloc.c
index 64263756ea2ccb2a5391e028a449fffae4e66773..8bf73b262ccfad2cfc1089fc47267309b7baafce 100644
--- a/zmalloc.c
+++ b/zmalloc.c
@@ -40,7 +40,28 @@
 #define PREFIX_SIZE sizeof(size_t)
 #endif
 
+#define increment_used_memory(_n) do { \
+    if (zmalloc_thread_safe) { \
+        pthread_mutex_lock(&used_memory_mutex);  \
+        used_memory += _n; \
+        pthread_mutex_unlock(&used_memory_mutex); \
+    } else { \
+        used_memory += _n; \
+    } \
+} while(0)
+
+#define decrement_used_memory(_n) do { \
+    if (zmalloc_thread_safe) { \
+        pthread_mutex_lock(&used_memory_mutex);  \
+        used_memory -= _n; \
+        pthread_mutex_unlock(&used_memory_mutex); \
+    } else { \
+        used_memory -= _n; \
+    } \
+} while(0)
+
 static size_t used_memory = 0;
+static int zmalloc_thread_safe = 0;
 pthread_mutex_t used_memory_mutex = PTHREAD_MUTEX_INITIALIZER;
 
 static void zmalloc_oom(size_t size) {
@@ -55,11 +76,11 @@ void *zmalloc(size_t size) {
 
     if (!ptr) zmalloc_oom(size);
 #ifdef HAVE_MALLOC_SIZE
-    used_memory += redis_malloc_size(ptr);
+    increment_used_memory(redis_malloc_size(ptr));
     return ptr;
 #else
     *((size_t*)ptr) = size;
-    used_memory += size+PREFIX_SIZE;
+    increment_used_memory(size+PREFIX_SIZE);
     return (char*)ptr+PREFIX_SIZE;
 #endif
 }
@@ -77,8 +98,8 @@ void *zrealloc(void *ptr, size_t size) {
     newptr = realloc(ptr,size);
     if (!newptr) zmalloc_oom(size);
 
-    used_memory -= oldsize;
-    used_memory += redis_malloc_size(newptr);
+    decrement_used_memory(oldsize);
+    increment_used_memory(redis_malloc_size(newptr));
     return newptr;
 #else
     realptr = (char*)ptr-PREFIX_SIZE;
@@ -87,8 +108,8 @@ void *zrealloc(void *ptr, size_t size) {
     if (!newptr) zmalloc_oom(size);
 
     *((size_t*)newptr) = size;
-    used_memory -= oldsize;
-    used_memory += size;
+    decrement_used_memory(oldsize);
+    increment_used_memory(size);
     return (char*)newptr+PREFIX_SIZE;
 #endif
 }
@@ -101,12 +122,12 @@ void zfree(void *ptr) {
 
     if (ptr == NULL) return;
 #ifdef HAVE_MALLOC_SIZE
-    used_memory -= redis_malloc_size(ptr);
+    decrement_used_memory(redis_malloc_size(ptr));
     free(ptr);
 #else
     realptr = (char*)ptr-PREFIX_SIZE;
     oldsize = *((size_t*)realptr);
-    used_memory -= oldsize+PREFIX_SIZE;
+    decrement_used_memory(oldsize+PREFIX_SIZE);
     free(realptr);
 #endif
 }
@@ -120,5 +141,14 @@ char *zstrdup(const char *s) {
 }
 
 size_t zmalloc_used_memory(void) {
-    return used_memory;
+    size_t um;
+
+    if (zmalloc_thread_safe) pthread_mutex_lock(&used_memory_mutex);
+    um = used_memory;
+    if (zmalloc_thread_safe) pthread_mutex_unlock(&used_memory_mutex);
+    return um;
+}
+
+void zmalloc_enable_thread_safeness(void) {
+    zmalloc_thread_safe = 1;
 }
diff --git a/zmalloc.h b/zmalloc.h
index 9e70e8778c5a46c3821a42ce51ec2d78ab4cb758..fbe790302a301fee07daf0bd0c3b5e6920ab0399 100644
--- a/zmalloc.h
+++ b/zmalloc.h
@@ -36,5 +36,6 @@ void *zrealloc(void *ptr, size_t size);
 void zfree(void *ptr);
 char *zstrdup(const char *s);
 size_t zmalloc_used_memory(void);
+void zmalloc_enable_thread_safeness(void);
 
 #endif /* _ZMALLOC_H */