diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 84f0d76b988..e344f1c1fa6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -228,6 +228,7 @@ jobs:
           - powerpc64-unknown-linux-gnu
           - mips-unknown-linux-gnu
           - arm-linux-androideabi
+          - mipsel-unknown-linux-musl
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust ${{ env.rust_stable }}
@@ -241,7 +242,14 @@ jobs:
         with:
           use-cross: true
           command: check
-          args: --workspace --target ${{ matrix.target }}
+          args: --workspace --all-features --target ${{ matrix.target }}
+      - uses: actions-rs/cargo@v1
+        with:
+          use-cross: true
+          command: check
+          args: --workspace --all-features --target ${{ matrix.target }}
+        env:
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings
 
   features:
     name: features
diff --git a/Cross.toml b/Cross.toml
new file mode 100644
index 00000000000..050f2bdbd75
--- /dev/null
+++ b/Cross.toml
@@ -0,0 +1,4 @@
+[build.env]
+passthrough = [
+    "RUSTFLAGS",
+]
diff --git a/tokio/src/loom/std/atomic_u64.rs b/tokio/src/loom/std/atomic_u64.rs
index 113992d9775..ac20f352943 100644
--- a/tokio/src/loom/std/atomic_u64.rs
+++ b/tokio/src/loom/std/atomic_u64.rs
@@ -75,4 +75,12 @@ cfg_not_has_atomic_u64! {
             self.compare_exchange(current, new, success, failure)
         }
     }
+
+    impl Default for AtomicU64 {
+        fn default() -> AtomicU64 {
+            Self {
+                inner: Mutex::new(0),
+            }
+        }
+    }
 }
diff --git a/tokio/src/runtime/metrics/io.rs b/tokio/src/runtime/metrics/io.rs
index 4928c48e8d8..9706bfc9bc2 100644
--- a/tokio/src/runtime/metrics/io.rs
+++ b/tokio/src/runtime/metrics/io.rs
@@ -1,6 +1,6 @@
 #![cfg_attr(not(feature = "net"), allow(dead_code))]
 
-use std::sync::atomic::{AtomicU64, Ordering::Relaxed};
+use crate::loom::sync::atomic::{AtomicU64, Ordering::Relaxed};
 
 #[derive(Default)]
 pub(crate) struct IoDriverMetrics {