diff --git a/src/libcore/os.rs b/src/libcore/os.rs
index 93319efa3b76..a198b495127e 100644
--- a/src/libcore/os.rs
+++ b/src/libcore/os.rs
@@ -147,23 +147,25 @@ pub mod win32 {
 
 /*
 Accessing environment variables is not generally threadsafe.
-This uses a per-runtime lock to serialize access.
-FIXME #4726: It would probably be appropriate to make this a real global
+Serialize access through a global lock.
 */
 fn with_env_lock<T>(f: &fn() -> T) -> T {
-    use unstable::global::global_data_clone_create;
-    use unstable::sync::{Exclusive, exclusive};
-
-    struct SharedValue(());
-    type ValueMutex = Exclusive<SharedValue>;
-    fn key(_: ValueMutex) { }
+    use unstable::finally::Finally;
 
     unsafe {
-        let lock: ValueMutex = global_data_clone_create(key, || {
-            ~exclusive(SharedValue(()))
-        });
+        return do (|| {
+            rust_take_env_lock();
+            f()
+        }).finally {
+            rust_drop_env_lock();
+        };
+    }
 
-        lock.with_imm(|_| f() )
+    extern {
+        #[fast_ffi]
+        fn rust_take_env_lock();
+        #[fast_ffi]
+        fn rust_drop_env_lock();
     }
 }
 
diff --git a/src/libuv b/src/libuv
index 97ac7c087a0c..218ab86721ee 160000
--- a/src/libuv
+++ b/src/libuv
@@ -1 +1 @@
-Subproject commit 97ac7c087a0caf6b0f611b80e14f7fe3cb18bb27
+Subproject commit 218ab86721eefd7b7e97fa6d9f95a80a1fa8686c
diff --git a/src/rt/rust_env.cpp b/src/rt/rust_env.cpp
index 360d61149285..ed38be3550f7 100644
--- a/src/rt/rust_env.cpp
+++ b/src/rt/rust_env.cpp
@@ -13,6 +13,7 @@
 // that might come from the environment is loaded here, once, during
 // init.
 
+#include "sync/lock_and_signal.h"
 #include "rust_env.h"
 
 // The environment variables that the runtime knows about
@@ -26,6 +27,18 @@
 #define RUST_DEBUG_MEM "RUST_DEBUG_MEM"
 #define RUST_DEBUG_BORROW "RUST_DEBUG_BORROW"
 
+static lock_and_signal env_lock;
+
+extern "C" CDECL void
+rust_take_env_lock() {
+    env_lock.lock();
+}
+
+extern "C" CDECL void
+rust_drop_env_lock() {
+    env_lock.unlock();
+}
+
 #if defined(__WIN32__)
 static int
 get_num_cpus() {
@@ -119,6 +132,8 @@ copyenv(const char* name) {
 
 rust_env*
 load_env(int argc, char **argv) {
+    scoped_lock with(env_lock);
+
     rust_env *env = (rust_env*)malloc(sizeof(rust_env));
 
     env->num_sched_threads = (size_t)get_num_threads();
@@ -141,3 +156,4 @@ free_env(rust_env *env) {
     free(env->rust_seed);
     free(env);
 }
+
diff --git a/src/rt/rustrt.def.in b/src/rt/rustrt.def.in
index a62d7991d495..958b31eb3429 100644
--- a/src/rt/rustrt.def.in
+++ b/src/rt/rustrt.def.in
@@ -235,4 +235,7 @@ rust_begin_unwind
 rust_take_task_borrow_list
 rust_set_task_borrow_list
 rust_valgrind_stack_register
-rust_valgrind_stack_deregister
\ No newline at end of file
+rust_valgrind_stack_deregister
+rust_take_env_lock
+rust_drop_env_lock
+