diff --git a/src/libcore/sync.rs b/src/libcore/sync.rs
index 3e1b85441e9..909d4d4dc1b 100644
--- a/src/libcore/sync.rs
+++ b/src/libcore/sync.rs
@@ -5,11 +5,17 @@
  * in std.
  */
 
+export condvar;
 export semaphore, new_semaphore;
+export mutex, new_mutex;
 
 // FIXME (#3119) This shouldn't be a thing exported from core.
 import arc::exclusive;
 
+/****************************************************************************
+ * Internals
+ ****************************************************************************/
+
 // Each waiting task receives on one of these. FIXME #3125 make these oneshot.
 type wait_end = pipes::port<()>;
 type signal_end = pipes::chan<()>;
@@ -17,42 +23,24 @@ type signal_end = pipes::chan<()>;
 type waitqueue = { head: pipes::port<signal_end>,
                    tail: pipes::chan<signal_end> };
 
-fn waitqueue() -> waitqueue {
-    let (tail, head) = pipes::stream();
-    { head: head, tail: tail }
-}
-
-/// A counting, blocking, bounded-waiting semaphore.
-enum semaphore = exclusive<semaphore_inner>;
-type semaphore_inner = {
+// The building-block used to make semaphores, lock-and-signals, and rwlocks.
+enum sem<Q: send> = exclusive<{
     mut count: int,
     waiters:   waitqueue,
-    //blocked:   waitqueue,
-};
+    // Can be either unit or another waitqueue. Some sems shouldn't come with
+    // a condition variable attached, others should.
+    blocked:   Q,
+}>;
 
-/// Create a new semaphore with the specified count.
-fn new_semaphore(count: int) -> semaphore {
-    semaphore(exclusive({ mut count: count,
-                          waiters: waitqueue(), /* blocked: waitqueue() */ }))
-}
-
-impl semaphore for &semaphore {
-    /// Creates a new handle to the semaphore.
-    fn clone() -> semaphore {
-        semaphore((**self).clone())
-    }
-
-    /**
-     * Acquires a resource represented by the semaphore. Blocks if necessary
-     * until resource(s) become available.
-     */
+impl sem<Q: send> for &sem<Q> {
     fn acquire() {
         let mut waiter_nobe = none;
         unsafe {
             do (**self).with |state| {
                 state.count -= 1;
                 if state.count < 0 {
-                    let (signal_end,wait_end) = pipes::stream();
+                    let (signal_end, wait_end) = pipes::stream();
+                    // Tell outer scope we need to block.
                     waiter_nobe = some(wait_end);
                     // Enqueue ourself.
                     state.waiters.tail.send(signal_end);
@@ -66,42 +54,192 @@ impl semaphore for &semaphore {
             let _ = option::unwrap(waiter_nobe).recv();
         }
     }
-
-    /**
-     * Release a held resource represented by the semaphore. Wakes a blocked
-     * contending task, if any exist.
-     */
     fn release() {
         unsafe {
             do (**self).with |state| {
                 state.count += 1;
                 // The peek is mandatory to make sure recv doesn't block.
-                if state.count >= 0 && state.waiters.head.peek() {
+                if state.count <= 0 && state.waiters.head.peek() {
                     // Pop off the waitqueue and send a wakeup signal. If the
                     // waiter was killed, its port will have closed, and send
                     // will fail. Keep trying until we get a live task.
                     state.waiters.head.recv().send(());
-                    // to-do: use this version when it's ready, kill-friendly.
+                    // FIXME(#3145) use kill-friendly version when ready
                     // while !state.waiters.head.recv().try_send(()) { }
                 }
             }
         }
     }
-
-    /// Runs a function with ownership of one of the semaphore's resources.
+}
+// FIXME(#3154) move both copies of this into sem<Q>, and unify the 2 structs
+impl sem_access for &sem<()> {
     fn access<U>(blk: fn() -> U) -> U {
         self.acquire();
         let _x = sem_release(self);
         blk()
     }
 }
+impl sem_access for &sem<waitqueue> {
+    fn access<U>(blk: fn() -> U) -> U {
+        self.acquire();
+        let _x = sem_and_signal_release(self);
+        blk()
+    }
+}
 
 // FIXME(#3136) should go inside of access()
 struct sem_release {
-    sem: &semaphore;
-    new(sem: &semaphore) { self.sem = sem; }
+    sem: &sem<()>;
+    new(sem: &sem<()>) { self.sem = sem; }
     drop { self.sem.release(); }
 }
+struct sem_and_signal_release {
+    sem: &sem<waitqueue>;
+    new(sem: &sem<waitqueue>) { self.sem = sem; }
+    drop { self.sem.release(); }
+}
+
+/// A mechanism for atomic-unlock-and-deschedule blocking and signalling.
+enum condvar = &sem<waitqueue>;
+
+impl condvar for condvar {
+    /// Atomically drop the associated lock, and block until a signal is sent.
+    fn wait() {
+        let (signal_end, wait_end) = pipes::stream();
+        let mut signal_end = some(signal_end);
+        unsafe {
+            do (***self).with |state| {
+                // Drop the lock.
+                // FIXME(#3145) investigate why factoring doesn't compile.
+                state.count += 1;
+                if state.count <= 0 && state.waiters.head.peek() {
+                    state.waiters.head.recv().send(());
+                    // FIXME(#3145) use kill-friendly version when ready
+                }
+                // Enqueue ourself to be woken up by a signaller.
+                state.blocked.tail.send(option::swap_unwrap(&mut signal_end));
+            }
+        }
+        // Unconditionally "block". (Might not actually block if a signaller
+        // did send -- I mean 'unconditionally' in contrast with acquire().)
+        let _ = wait_end.recv();
+        // Pick up the lock again. FIXME(#3145): unkillable? destructor?
+        (*self).acquire();
+    }
+
+    /// Wake up a blocked task. Returns false if there was no blocked task.
+    fn signal() -> bool {
+        unsafe {
+            do (***self).with |state| {
+                if state.blocked.head.peek() {
+                    state.blocked.head.recv().send(());
+                    // FIXME(#3145) use kill-friendly version when ready
+                    true
+                } else {
+                    false
+                }
+            }
+        }
+    }
+
+    /// Wake up all blocked tasks. Returns the number of tasks woken.
+    fn broadcast() -> uint {
+        unsafe {
+            do (***self).with |state| {
+                let mut count = 0;
+                while state.blocked.head.peek() {
+                    // This is already kill-friendly.
+                    state.blocked.head.recv().send(());
+                    count += 1;
+                }
+                count
+            }
+        }
+    }
+}
+
+impl sem_and_signal for &sem<waitqueue> {
+    fn access_cond<U>(blk: fn(condvar) -> U) -> U {
+        do self.access { blk(condvar(self)) }
+    }
+}
+
+/****************************************************************************
+ * Semaphores
+ ****************************************************************************/
+
+/// A counting, blocking, bounded-waiting semaphore.
+enum semaphore = sem<()>;
+
+/// Create a new semaphore with the specified count.
+fn new_semaphore(count: int) -> semaphore {
+    let (wait_tail, wait_head)  = pipes::stream();
+    semaphore(sem(exclusive({ mut count: count,
+                              waiters: { head: wait_head, tail: wait_tail },
+                              blocked: () })))
+}
+
+impl semaphore for &semaphore {
+    /// Create a new handle to the semaphore.
+    fn clone() -> semaphore { semaphore(sem((***self).clone())) }
+
+    /**
+     * Acquire a resource represented by the semaphore. Blocks if necessary
+     * until resource(s) become available.
+     */
+    fn acquire() { (&**self).acquire() }
+
+    /**
+     * Release a held resource represented by the semaphore. Wakes a blocked
+     * contending task, if any exist.
+     */
+    fn release() { (&**self).release() }
+
+    /// Run a function with ownership of one of the semaphore's resources.
+    fn access<U>(blk: fn() -> U) -> U { (&**self).access(blk) }
+}
+
+/****************************************************************************
+ * Mutexes
+ ****************************************************************************/
+
+/**
+ * A blocking, bounded-waiting, mutual exclusion lock with an associated
+ * FIFO condition variable.
+ */
+enum mutex = sem<waitqueue>;
+
+/// Create a new mutex.
+fn new_mutex() -> mutex {
+    let (wait_tail,  wait_head)  = pipes::stream();
+    let (block_tail, block_head) = pipes::stream();
+    mutex(sem(exclusive({ mut count: 1,
+                          waiters: { head: wait_head,  tail: wait_tail  },
+                          blocked: { head: block_head, tail: block_tail } })))
+}
+
+impl mutex for &mutex {
+    /// Create a new handle to the mutex.
+    fn clone() -> mutex { mutex(sem((***self).clone())) }
+
+    /// Run a function with ownership of the mutex.
+    fn lock<U>(blk: fn() -> U) -> U { (&**self).access(blk) }
+
+    /// Run a function with ownership of the mutex and a handle to a condvar.
+    fn lock_cond<U>(blk: fn(condvar) -> U) -> U {
+        (&**self).access_cond(blk)
+    }
+}
+
+/****************************************************************************
+ * Reader-writer locks
+ ****************************************************************************/
+
+// FIXME(#3145) implement
+
+/****************************************************************************
+ * Tests
+ ****************************************************************************/
 
 #[cfg(test)]
 mod tests {
@@ -145,35 +283,6 @@ mod tests {
         c.send(());
     }
     #[test]
-    fn test_sem_mutual_exclusion() {
-        // Unsafely achieve shared state, and do the textbook
-        // "load tmp <- ptr; inc tmp; store ptr <- tmp" dance.
-        let (c,p) = pipes::stream();
-        let s = ~new_semaphore(1);
-        let s2 = ~s.clone();
-        let sharedstate = ~0;
-        let ptr = ptr::addr_of(*sharedstate);
-        do task::spawn {
-            let sharedstate = unsafe { unsafe::reinterpret_cast(ptr) };
-            access_shared(sharedstate, s2, 10);
-            c.send(());
-        }
-        access_shared(sharedstate, s, 10);
-        let _ = p.recv();
-
-        assert *sharedstate == 20;
-
-        fn access_shared(sharedstate: &mut int, sem: &semaphore, n: uint) {
-            for n.times {
-                do sem.access {
-                    let oldval = *sharedstate;
-                    task::yield();
-                    *sharedstate = oldval + 1;
-                }
-            }
-        }
-    }
-    #[test]
     fn test_sem_multi_resource() {
         // Parent and child both get in the critical section at the same
         // time, and shake hands.
@@ -214,4 +323,90 @@ mod tests {
             let _ = p.recv(); // wait for child to be done
         }
     }
+    #[test]
+    fn test_mutex() {
+        // Unsafely achieve shared state, and do the textbook
+        // "load tmp <- ptr; inc tmp; store ptr <- tmp" dance.
+        let (c,p) = pipes::stream();
+        let m = ~new_mutex();
+        let m2 = ~m.clone();
+        let sharedstate = ~0;
+        let ptr = ptr::addr_of(*sharedstate);
+        do task::spawn {
+            let sharedstate = unsafe { unsafe::reinterpret_cast(ptr) };
+            access_shared(sharedstate, m2, 10);
+            c.send(());
+        }
+        access_shared(sharedstate, m, 10);
+        let _ = p.recv();
+
+        assert *sharedstate == 20;
+
+        fn access_shared(sharedstate: &mut int, sem: &mutex, n: uint) {
+            for n.times {
+                do sem.lock {
+                    let oldval = *sharedstate;
+                    task::yield();
+                    *sharedstate = oldval + 1;
+                }
+            }
+        }
+    }
+    #[test]
+    fn test_mutex_cond_wait() {
+        let m = ~new_mutex();
+        let mut m2 = some(~m.clone());
+
+        // Child wakes up parent
+        do m.lock_cond |cond| {
+            let m2 = option::swap_unwrap(&mut m2);
+            do task::spawn {
+                do m2.lock_cond |cond| { cond.signal(); }
+            }
+            cond.wait();
+        }
+        // Parent wakes up child
+        let (chan,port) = pipes::stream();
+        let m3 = ~m.clone();
+        do task::spawn {
+            do m3.lock_cond |cond| {
+                chan.send(());
+                cond.wait();
+                chan.send(());
+            }
+        }
+        let _ = port.recv(); // Wait until child gets in the mutex
+        do m.lock_cond |cond| {
+            cond.signal();
+        }
+        let _ = port.recv(); // Wait until child wakes up
+    }
+    #[test]
+    fn test_mutex_cond_broadcast() {
+        let num_waiters: uint = 12;
+        let m = ~new_mutex();
+        let mut ports = ~[];
+
+        for num_waiters.times {
+            let mi = ~m.clone();
+            let (chan, port) = pipes::stream();
+            vec::push(ports, port);
+            do task::spawn {
+                do mi.lock_cond |cond| {
+                    chan.send(());
+                    cond.wait();
+                    chan.send(());
+                }
+            }
+        }
+
+        // wait until all children get in the mutex
+        for ports.each |port| { let _ = port.recv(); }
+        do m.lock_cond |cond| {
+            let num_woken = cond.broadcast();
+            assert num_woken == num_waiters;
+        }
+        // wait until all children wake up
+        for ports.each |port| { let _ = port.recv(); }
+    }
 }