diff --git a/src/libextra/rl.rs b/src/libextra/rl.rs
index 09ef7f22be5..f4ad07d464d 100644
--- a/src/libextra/rl.rs
+++ b/src/libextra/rl.rs
@@ -8,13 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// FIXME #3921. This is unsafe because linenoise uses global mutable
-// state without mutexes.
-
 use std::c_str::ToCStr;
 use std::libc::{c_char, c_int};
-use std::local_data;
-use std::str;
+use std::{local_data, str, rt};
+use std::unstable::finally::Finally;
 
 #[cfg(stage0)]
 pub mod rustrt {
@@ -28,6 +25,9 @@ pub mod rustrt {
         fn linenoiseHistoryLoad(file: *c_char) -> c_int;
         fn linenoiseSetCompletionCallback(callback: *u8);
         fn linenoiseAddCompletion(completions: *(), line: *c_char);
+
+        fn rust_take_linenoise_lock();
+        fn rust_drop_linenoise_lock();
     }
 }
 
@@ -42,38 +42,53 @@ pub mod rustrt {
     externfn!(fn linenoiseHistoryLoad(file: *c_char) -> c_int)
     externfn!(fn linenoiseSetCompletionCallback(callback: extern "C" fn(*i8, *())))
     externfn!(fn linenoiseAddCompletion(completions: *(), line: *c_char))
+
+    externfn!(fn rust_take_linenoise_lock())
+    externfn!(fn rust_drop_linenoise_lock())
+}
+
+macro_rules! locked {
+    ($expr:expr) => {
+        unsafe {
+            // FIXME #9105: can't use a static mutex in pure Rust yet.
+            rustrt::rust_take_linenoise_lock();
+            let x = $expr;
+            rustrt::rust_drop_linenoise_lock();
+            x
+        }
+    }
 }
 
 /// Add a line to history
-pub unsafe fn add_history(line: &str) -> bool {
+pub fn add_history(line: &str) -> bool {
     do line.with_c_str |buf| {
-        rustrt::linenoiseHistoryAdd(buf) == 1 as c_int
+        (locked!(rustrt::linenoiseHistoryAdd(buf))) == 1 as c_int
     }
 }
 
 /// Set the maximum amount of lines stored
-pub unsafe fn set_history_max_len(len: int) -> bool {
-    rustrt::linenoiseHistorySetMaxLen(len as c_int) == 1 as c_int
+pub fn set_history_max_len(len: int) -> bool {
+    (locked!(rustrt::linenoiseHistorySetMaxLen(len as c_int))) == 1 as c_int
 }
 
 /// Save line history to a file
-pub unsafe fn save_history(file: &str) -> bool {
+pub fn save_history(file: &str) -> bool {
     do file.with_c_str |buf| {
-        rustrt::linenoiseHistorySave(buf) == 1 as c_int
+        (locked!(rustrt::linenoiseHistorySave(buf))) == 1 as c_int
     }
 }
 
 /// Load line history from a file
-pub unsafe fn load_history(file: &str) -> bool {
+pub fn load_history(file: &str) -> bool {
     do file.with_c_str |buf| {
-        rustrt::linenoiseHistoryLoad(buf) == 1 as c_int
+        (locked!(rustrt::linenoiseHistoryLoad(buf))) == 1 as c_int
     }
 }
 
 /// Print out a prompt and then wait for input and return it
-pub unsafe fn read(prompt: &str) -> Option<~str> {
+pub fn read(prompt: &str) -> Option<~str> {
     do prompt.with_c_str |buf| {
-        let line = rustrt::linenoise(buf);
+        let line = locked!(rustrt::linenoise(buf));
 
         if line.is_null() { None }
         else { Some(str::raw::from_c_str(line)) }
@@ -84,8 +99,13 @@ pub type CompletionCb = @fn(~str, @fn(~str));
 
 static complete_key: local_data::Key<@CompletionCb> = &local_data::Key;
 
-/// Bind to the main completion callback
-pub unsafe fn complete(cb: CompletionCb) {
+/// Bind to the main completion callback.
+///
+/// The completion callback should not call any `extra::rl` functions
+/// other than the closure that it receives as its second
+/// argument. Calling such a function will deadlock on the mutex used
+/// to ensure that the calls are thread-safe.
+pub fn complete(cb: CompletionCb) {
     local_data::set(complete_key, @cb);
 
     extern fn callback(line: *c_char, completions: *()) {
@@ -95,6 +115,10 @@ pub unsafe fn complete(cb: CompletionCb) {
             unsafe {
                 do cb(str::raw::from_c_str(line)) |suggestion| {
                     do suggestion.with_c_str |buf| {
+                        // This isn't locked, because `callback` gets
+                        // called inside `rustrt::linenoise`, which
+                        // *is* already inside the mutex, so
+                        // re-locking would be a deadlock.
                         rustrt::linenoiseAddCompletion(completions, buf);
                     }
                 }
@@ -102,5 +126,5 @@ pub unsafe fn complete(cb: CompletionCb) {
         }
     }
 
-    rustrt::linenoiseSetCompletionCallback(callback);
+    locked!(rustrt::linenoiseSetCompletionCallback(callback));
 }
diff --git a/src/librusti/rusti.rs b/src/librusti/rusti.rs
index 5bd941759f4..8d61a971157 100644
--- a/src/librusti/rusti.rs
+++ b/src/librusti/rusti.rs
@@ -355,12 +355,12 @@ fn compile_crate(src_filename: ~str, binary: ~str) -> Option<bool> {
 /// None if no input was read (e.g. EOF was reached).
 fn get_line(use_rl: bool, prompt: &str) -> Option<~str> {
     if use_rl {
-        let result = unsafe { rl::read(prompt) };
+        let result = rl::read(prompt);
 
         match result {
             None => None,
             Some(line) => {
-                unsafe { rl::add_history(line) };
+                rl::add_history(line);
                 Some(line)
             }
         }
@@ -525,14 +525,12 @@ pub fn main_args(args: &[~str]) {
         println("unstable. If you encounter problems, please use the");
         println("compiler instead. Type :help for help.");
 
-        unsafe {
-            do rl::complete |line, suggest| {
-                if line.starts_with(":") {
-                    suggest(~":clear");
-                    suggest(~":exit");
-                    suggest(~":help");
-                    suggest(~":load");
-                }
+        do rl::complete |line, suggest| {
+            if line.starts_with(":") {
+                suggest(~":clear");
+                suggest(~":exit");
+                suggest(~":help");
+                suggest(~":load");
             }
         }
     }
diff --git a/src/rt/rust_builtin.cpp b/src/rt/rust_builtin.cpp
index 03a17d2c2ef..1871e7f36b3 100644
--- a/src/rt/rust_builtin.cpp
+++ b/src/rt/rust_builtin.cpp
@@ -633,6 +633,18 @@ rust_drop_env_lock() {
     env_lock.unlock();
 }
 
+static lock_and_signal linenoise_lock;
+
+extern "C" CDECL void
+rust_take_linenoise_lock() {
+    linenoise_lock.lock();
+}
+
+extern "C" CDECL void
+rust_drop_linenoise_lock() {
+    linenoise_lock.unlock();
+}
+
 extern "C" CDECL unsigned int
 rust_valgrind_stack_register(void *start, void *end) {
   return VALGRIND_STACK_REGISTER(start, end);