diff --git a/tracing-subscriber/src/layer.rs b/tracing-subscriber/src/layer.rs
index a56fad9b..53e6ef93 100644
--- a/tracing-subscriber/src/layer.rs
+++ b/tracing-subscriber/src/layer.rs
@@ -626,11 +626,11 @@ impl<'a, S: Subscriber + Registry> Context<'a, S> {
         self.subscriber.as_ref()?.span(id)
     }
 
-    fn span_mut(&self, id: &span::Id) -> Option<Self::RefMut> {
+    fn span_mut(&self, id: &span::Id) -> Option<S::RefMut> {
         self.subscriber.as_ref()?.span_mut(id)
     }
 
-    fn parents<'a>(&'a self, span: &span::Id) -> Parents<'a, S>
+    fn parents(&'a self, span: &span::Id) -> Parents<'a, S>
     where
         S: Sized,
     {
diff --git a/tracing-subscriber/src/registry/sharded.rs b/tracing-subscriber/src/registry/sharded.rs
new file mode 100644
index 00000000..8c870b1c
--- /dev/null
+++ b/tracing-subscriber/src/registry/sharded.rs
@@ -0,0 +1,285 @@
+use hashbrown::HashMap;
+use tracing_core::span::Id;
+use std::{
+    mem,
+    thread,
+    sync::{
+        atomic::{AtomicUsize, Ordering},
+        PoisonError,
+    },
+    cell::{RefCell, Cell, Ref, RefMut, UnsafeCell},
+    fmt,
+};
+use parking_lot::{ReentrantMutex, ReentrantMutexGuard, MappedReentrantMutexGuard};
+use crossbeam_utils::sync::{ShardedLock, ShardedLockReadGuard};
+use owning_ref::OwningHandle;
+
+pub struct Registry<T> {
+    shards: ShardedLock<Shards<T>>,
+}
+
+#[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
+struct Thread {
+    id: usize,
+}
+
+struct Shards<T>(HashMap<Thread, Shard<T>>);
+
+struct Shard<T> {
+    spans: ReentrantMutex<UnsafeCell<Spans<T>>>,
+}
+
+pub struct Span<'a, T> {
+    inner: MappedReentrantMutexGuard<'a, Ref<'a, T>>,
+}
+
+pub struct SpanMut<'a, T> {
+    inner: MappedReentrantMutexGuard<'a, RefMut<'a, T>>,
+}
+
+#[derive(Debug)]
+enum Error {
+    Poisoned,
+    BadInsert,
+}
+
+type Spans<T> = HashMap<Id, RefCell<T>>;
+
+fn handle_err<T>(result: Result<T, Error>) -> Option<T> {
+    match result {
+        Ok(e) => Some(e),
+        Err(e) if thread::panicking() => {
+            println!("{}", e);
+            None
+        },
+        Err(e) =>  panic!("{}", e),
+    }
+}
+
+impl<T> Registry<T> {
+    fn with_local<'a, I>(&'a self, mut f: impl FnOnce(&'a Shard<T>) -> I) -> Result<I, Error> {
+        // fast path --- the shard already exists
+        let thread = Thread::current();
+        let mut f = Some(f);
+
+        if let Some(r) = self.shards.read()?
+            .with_shard(&thread, &mut f)
+        {
+            Ok(r)
+        } else {
+            // slow path --- need to insert a shard.
+            self.with_new_local(&thread, &mut f)
+        }
+    }
+
+    #[cold]
+    fn with_new_local<'a, I>(&'a self, thread: &Thread, f: &mut Option<impl FnOnce(&'a Shard<T>)-> I>,) -> Result<I, Error> {
+        self.shards.write()?
+            .new_shard_for(thread.clone())
+            .with_shard(&thread, &mut f).ok_or(Error::BadInsert)
+    }
+
+    pub fn span(&self, id: &Id) -> Option<Span<T>> {
+        let local = self.with_local(|shard| { shard.span(id) });
+        let local = handle_err(local)?;
+        match local {
+            Some(slot) => {
+                let inner = MappedReentrantMutexGuard::try_map(slot, |slot| slot.as_ref()).ok()?;
+                return Some(Span { inner });
+            },
+            None => {
+                // TODO: steal
+            }
+        }
+
+        None
+    }
+
+    pub fn span_mut<'a>(&'a self, id: &Id) -> Option<SpanMut<'a, T>> {
+        // let res = self.with_local(|shard| {
+        //     shard.span(id).map(|inner| Ref { inner })
+        // });
+        // handle_err(res)?
+        unimplemented!()
+    }
+
+    pub fn with_span<I>(&self, id: &Id, f: impl FnOnce(&mut T) -> I) -> Option<I> {
+        // let mut f = Some(f);
+        // let res = self.with_shard(|shard| {
+        //     shard.get_mut(id).and_then(Slot::get_mut).map(|span| {
+        //         let mut f = f.take().expect("called twice!");
+        //         f(span)
+        //     })
+        // });
+        // handle_poison(res)?
+
+        // TODO: steal
+        unimplemented!()
+    }
+
+    pub fn insert(&self, id: Id, span: T) -> &Self {
+        let ok = self.with_local(move |shard| { shard.insert(id, span) });
+        if !thread::panicking() {
+            ok.expect("poisoned");
+        }
+
+        self
+    }
+
+    pub fn new() -> Self {
+        Self {
+            shards: ShardedLock::new(Shards(HashMap::new()))
+        }
+    }
+}
+
+impl<T> Shards<T> {
+    fn with_shard<'a, I>(
+        &'a self,
+        thread: &Thread,
+        f: &mut Option<impl FnOnce(&'a Shard<T>)-> I>,
+    ) -> Option<I> {
+        let shard = self.0.get(thread)?;
+        let mut f = match f.take() {
+            None if thread::panicking() => {
+                println!("with_shard: closure called twice; this is a bug");
+                return None;
+            },
+            None => panic!("with_shard: closure called twice; this is a bug"),
+            Some(f) => f,
+        };
+        Some(f(&*shard))
+    }
+
+    fn new_shard_for(&mut self, thread: Thread) -> &mut Self {
+        self.0.insert(thread, Shard::new());
+        self
+    }
+}
+
+impl<T> Shard<T> {
+    fn new() -> Self {
+        Self {
+            spans: ReentrantMutex::new(UnsafeCell::new(HashMap::new()))
+        }
+    }
+
+    fn insert(&self, id: Id, span: T) -> Option<T> {
+        let guard = self.spans.lock();
+        let spans = unsafe {
+            // this is safe as the mutex ensures that the map is not mutated
+            // concurrently, and the mutable ref will not leave this function.
+            &mut *(guard.get())
+        };
+        spans.insert(id, RefCell::new(span))
+    }
+
+    // fn spans_mut<'a>(&'a self) -> MappedReentrantMutexGuard<'a, RefMut<'a, Spans<T>>> {
+    //     let guard = self.spans.lock();
+    //     ReentrantMutexGuard::map(guard, RefCell::borrow_mut)
+    // }
+    fn span<'a>(&'a self, id: &Id) -> Option<MappedReentrantMutexGuard<'a, RefMut<T>>> {
+        let guard = self.spans.lock();
+        ReentrantMutexGuard::try_map(
+            guard,
+            move |spans| {
+                let spans = unsafe { &mut *(guard.get()) };
+                spans.get_mut(id)
+            }
+        ).ok()
+    }
+
+    fn try_steal(&self, id: &Id) -> Option<Slot<T>> {
+        let guard = self.spans.lock();
+        let spans = unsafe {
+            // this is safe as the mutex ensures that the map is not mutated
+            // concurrently, and the mutable ref will not leave this function.
+            &mut *(guard.get())
+        };
+        spans.insert(id.clone(), Slot::Stolen(Thread::current()))
+    }
+}
+
+impl Thread {
+    fn current() -> Self {
+        static NEXT: AtomicUsize = AtomicUsize::new(0);
+        thread_local! {
+            static MY_ID: Cell<Option<usize>> = Cell::new(None);
+        }
+        MY_ID.with(|my_id| if let Some(id) = my_id.get() {
+            Thread {
+                id
+            }
+        } else {
+            let id = NEXT.fetch_add(1, Ordering::SeqCst);
+            my_id.set(Some(id));
+            Thread {
+                id
+            }
+        })
+    }
+}
+
+impl<T> Slot<T> {
+
+    fn as_ref<'a>(&'a self) -> Option<Ref<'a, T>> {
+        match self {
+            Slot::Present(ref span) => Some(span.borrow()),
+            _ => None,
+        }
+    }
+
+    fn as_mut<'a>(&'a self) -> Option<RefMut<'a, T>> {
+        match self {
+            Slot::Present(ref span) => Some(span.borrow_mut()),
+            _ => None,
+        }
+    }
+
+    fn into_option(self) -> Option<T> {
+        match self {
+            Slot::Present(span) => Some(span.into_inner()),
+            _ => None,
+        }
+    }
+}
+
+
+impl<T> From<PoisonError<T>> for Error {
+    fn from(_: PoisonError<T>) -> Self {
+        Error::Poisoned
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Error::Poisoned =>
+                "registry poisoned: another thread panicked inside".fmt(f),
+            Error::BadInsert =>
+                "new thread was inserted but did not exist; this is a bug".fmt(f),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn basically_works() {
+        let registry: Registry<usize> = Registry::new();
+        registry
+            .insert(Id::from_u64(1), 1)
+            .insert(Id::from_u64(2), 2);
+
+        assert_eq!(registry.span(&Id::from_u64(1)), Some(1));
+        assert_eq!(registry.span(&Id::from_u64(2)), Some(2));
+
+        registry.insert(Id::from_u64(3), 3);
+
+        assert_eq!(registry.span(&Id::from_u64(1)), Some(1));
+        assert_eq!(registry.span(&Id::from_u64(2)), Some(2));
+        assert_eq!(registry.span(&Id::from_u64(3)), Some(3));
+    }
+}