Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 1493 insertions, 0 deletions

diff --git a/vendor/sharded-slab/src/page/mod.rs b/vendor/sharded-slab/src/page/mod.rs
new file mode 100644
index 000000000..0499fb535
--- /dev/null
+++ b/vendor/sharded-slab/src/page/mod.rs
@@ -0,0 +1,449 @@
+use crate::cfg::{self, CfgPrivate};
+use crate::clear::Clear;
+use crate::sync::UnsafeCell;
+use crate::Pack;
+
+pub(crate) mod slot;
+mod stack;
+pub(crate) use self::slot::Slot;
+use std::{fmt, marker::PhantomData};
+
+/// A page address encodes the location of a slot within a shard (the page
+/// number and offset within that page) as a single linear value.
+#[repr(transparent)]
+pub(crate) struct Addr<C: cfg::Config = cfg::DefaultConfig> {
+    addr: usize,
+    _cfg: PhantomData<fn(C)>,
+}
+
+impl<C: cfg::Config> Addr<C> {
+    const NULL: usize = Self::BITS + 1;
+
+    pub(crate) fn index(self) -> usize {
+        // Since every page is twice as large as the previous page, and all page sizes
+        // are powers of two, we can determine the page index that contains a given
+        // address by counting leading zeros, which tells us what power of two
+        // the offset fits into.
+        //
+        // First, we must shift down to the smallest page size, so that the last
+        // offset on the first page becomes 0.
+        let shifted = (self.addr + C::INITIAL_SZ) >> C::ADDR_INDEX_SHIFT;
+        // Now, we can  determine the number of twos places by counting the
+        // number of leading  zeros (unused twos places) in the number's binary
+        // representation, and subtracting that count from the total number of bits in a word.
+        cfg::WIDTH - shifted.leading_zeros() as usize
+    }
+
+    pub(crate) fn offset(self) -> usize {
+        self.addr
+    }
+}
+
+pub(crate) trait FreeList<C> {
+    fn push<T>(&self, new_head: usize, slot: &Slot<T, C>)
+    where
+        C: cfg::Config;
+}
+
+impl<C: cfg::Config> Pack<C> for Addr<C> {
+    const LEN: usize = C::MAX_PAGES + C::ADDR_INDEX_SHIFT;
+
+    type Prev = ();
+
+    fn as_usize(&self) -> usize {
+        self.addr
+    }
+
+    fn from_usize(addr: usize) -> Self {
+        debug_assert!(addr <= Self::BITS);
+        Self {
+            addr,
+            _cfg: PhantomData,
+        }
+    }
+}
+
+pub(crate) type Iter<'a, T, C> = std::iter::FilterMap<
+    std::slice::Iter<'a, Slot<Option<T>, C>>,
+    fn(&'a Slot<Option<T>, C>) -> Option<&'a T>,
+>;
+
+pub(crate) struct Local {
+    /// Index of the first slot on the local free list
+    head: UnsafeCell<usize>,
+}
+
+pub(crate) struct Shared<T, C> {
+    /// The remote free list
+    ///
+    /// Slots freed from a remote thread are pushed onto this list.
+    remote: stack::TransferStack<C>,
+    // Total size of the page.
+    //
+    // If the head index of the local or remote free list is greater than the size of the
+    // page, then that free list is emtpy. If the head of both free lists is greater than `size`
+    // then there are no slots left in that page.
+    size: usize,
+    prev_sz: usize,
+    slab: UnsafeCell<Option<Slots<T, C>>>,
+}
+
+type Slots<T, C> = Box<[Slot<T, C>]>;
+
+impl Local {
+    pub(crate) fn new() -> Self {
+        Self {
+            head: UnsafeCell::new(0),
+        }
+    }
+
+    #[inline(always)]
+    fn head(&self) -> usize {
+        self.head.with(|head| unsafe { *head })
+    }
+
+    #[inline(always)]
+    fn set_head(&self, new_head: usize) {
+        self.head.with_mut(|head| unsafe {
+            *head = new_head;
+        })
+    }
+}
+
+impl<C: cfg::Config> FreeList<C> for Local {
+    fn push<T>(&self, new_head: usize, slot: &Slot<T, C>) {
+        slot.set_next(self.head());
+        self.set_head(new_head);
+    }
+}
+
+impl<T, C> Shared<T, C>
+where
+    C: cfg::Config,
+{
+    const NULL: usize = Addr::<C>::NULL;
+
+    pub(crate) fn new(size: usize, prev_sz: usize) -> Self {
+        Self {
+            prev_sz,
+            size,
+            remote: stack::TransferStack::new(),
+            slab: UnsafeCell::new(None),
+        }
+    }
+
+    /// Return the head of the freelist
+    ///
+    /// If there is space on the local list, it returns the head of the local list. Otherwise, it
+    /// pops all the slots from the global list and returns the head of that list
+    ///
+    /// *Note*: The local list's head is reset when setting the new state in the slot pointed to be
+    /// `head` returned from this function
+    #[inline]
+    fn pop(&self, local: &Local) -> Option<usize> {
+        let head = local.head();
+
+        test_println!("-> local head {:?}", head);
+
+        // are there any items on the local free list? (fast path)
+        let head = if head < self.size {
+            head
+        } else {
+            // slow path: if the local free list is empty, pop all the items on
+            // the remote free list.
+            let head = self.remote.pop_all();
+
+            test_println!("-> remote head {:?}", head);
+            head?
+        };
+
+        // if the head is still null, both the local and remote free lists are
+        // empty --- we can't fit any more items on this page.
+        if head == Self::NULL {
+            test_println!("-> NULL! {:?}", head);
+            None
+        } else {
+            Some(head)
+        }
+    }
+
+    /// Returns `true` if storage is currently allocated for this page, `false`
+    /// otherwise.
+    #[inline]
+    fn is_unallocated(&self) -> bool {
+        self.slab.with(|s| unsafe { (*s).is_none() })
+    }
+
+    #[inline]
+    pub(crate) fn with_slot<'a, U>(
+        &'a self,
+        addr: Addr<C>,
+        f: impl FnOnce(&'a Slot<T, C>) -> Option<U>,
+    ) -> Option<U> {
+        let poff = addr.offset() - self.prev_sz;
+
+        test_println!("-> offset {:?}", poff);
+
+        self.slab.with(|slab| {
+            let slot = unsafe { &*slab }.as_ref()?.get(poff)?;
+            f(slot)
+        })
+    }
+
+    #[inline(always)]
+    pub(crate) fn free_list(&self) -> &impl FreeList<C> {
+        &self.remote
+    }
+}
+
+impl<'a, T, C> Shared<Option<T>, C>
+where
+    C: cfg::Config + 'a,
+{
+    pub(crate) fn take<F>(
+        &self,
+        addr: Addr<C>,
+        gen: slot::Generation<C>,
+        free_list: &F,
+    ) -> Option<T>
+    where
+        F: FreeList<C>,
+    {
+        let offset = addr.offset() - self.prev_sz;
+
+        test_println!("-> take: offset {:?}", offset);
+
+        self.slab.with(|slab| {
+            let slab = unsafe { &*slab }.as_ref()?;
+            let slot = slab.get(offset)?;
+            slot.remove_value(gen, offset, free_list)
+        })
+    }
+
+    pub(crate) fn remove<F: FreeList<C>>(
+        &self,
+        addr: Addr<C>,
+        gen: slot::Generation<C>,
+        free_list: &F,
+    ) -> bool {
+        let offset = addr.offset() - self.prev_sz;
+
+        test_println!("-> offset {:?}", offset);
+
+        self.slab.with(|slab| {
+            let slab = unsafe { &*slab }.as_ref();
+            if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
+                slot.try_remove_value(gen, offset, free_list)
+            } else {
+                false
+            }
+        })
+    }
+
+    // Need this function separately, as we need to pass a function pointer to `filter_map` and
+    // `Slot::value` just returns a `&T`, specifically a `&Option<T>` for this impl.
+    fn make_ref(slot: &'a Slot<Option<T>, C>) -> Option<&'a T> {
+        slot.value().as_ref()
+    }
+
+    pub(crate) fn iter(&self) -> Option<Iter<'a, T, C>> {
+        let slab = self.slab.with(|slab| unsafe { (&*slab).as_ref() });
+        slab.map(|slab| {
+            slab.iter()
+                .filter_map(Shared::make_ref as fn(&'a Slot<Option<T>, C>) -> Option<&'a T>)
+        })
+    }
+}
+
+impl<T, C> Shared<T, C>
+where
+    T: Clear + Default,
+    C: cfg::Config,
+{
+    pub(crate) fn init_with<U>(
+        &self,
+        local: &Local,
+        init: impl FnOnce(usize, &Slot<T, C>) -> Option<U>,
+    ) -> Option<U> {
+        let head = self.pop(local)?;
+
+        // do we need to allocate storage for this page?
+        if self.is_unallocated() {
+            self.allocate();
+        }
+
+        let index = head + self.prev_sz;
+
+        let result = self.slab.with(|slab| {
+            let slab = unsafe { &*(slab) }
+                .as_ref()
+                .expect("page must have been allocated to insert!");
+            let slot = &slab[head];
+            let result = init(index, slot)?;
+            local.set_head(slot.next());
+            Some(result)
+        })?;
+
+        test_println!("-> init_with: insert at offset: {}", index);
+        Some(result)
+    }
+
+    /// Allocates storage for the page's slots.
+    #[cold]
+    fn allocate(&self) {
+        test_println!("-> alloc new page ({})", self.size);
+        debug_assert!(self.is_unallocated());
+
+        let mut slab = Vec::with_capacity(self.size);
+        slab.extend((1..self.size).map(Slot::new));
+        slab.push(Slot::new(Self::NULL));
+        self.slab.with_mut(|s| {
+            // safety: this mut access is safe — it only occurs to initially allocate the page,
+            // which only happens on this thread; if the page has not yet been allocated, other
+            // threads will not try to access it yet.
+            unsafe {
+                *s = Some(slab.into_boxed_slice());
+            }
+        });
+    }
+
+    pub(crate) fn mark_clear<F: FreeList<C>>(
+        &self,
+        addr: Addr<C>,
+        gen: slot::Generation<C>,
+        free_list: &F,
+    ) -> bool {
+        let offset = addr.offset() - self.prev_sz;
+
+        test_println!("-> offset {:?}", offset);
+
+        self.slab.with(|slab| {
+            let slab = unsafe { &*slab }.as_ref();
+            if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
+                slot.try_clear_storage(gen, offset, free_list)
+            } else {
+                false
+            }
+        })
+    }
+
+    pub(crate) fn clear<F: FreeList<C>>(
+        &self,
+        addr: Addr<C>,
+        gen: slot::Generation<C>,
+        free_list: &F,
+    ) -> bool {
+        let offset = addr.offset() - self.prev_sz;
+
+        test_println!("-> offset {:?}", offset);
+
+        self.slab.with(|slab| {
+            let slab = unsafe { &*slab }.as_ref();
+            if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
+                slot.clear_storage(gen, offset, free_list)
+            } else {
+                false
+            }
+        })
+    }
+}
+
+impl fmt::Debug for Local {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.head.with(|head| {
+            let head = unsafe { *head };
+            f.debug_struct("Local")
+                .field("head", &format_args!("{:#0x}", head))
+                .finish()
+        })
+    }
+}
+
+impl<C, T> fmt::Debug for Shared<C, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Shared")
+            .field("remote", &self.remote)
+            .field("prev_sz", &self.prev_sz)
+            .field("size", &self.size)
+            // .field("slab", &self.slab)
+            .finish()
+    }
+}
+
+impl<C: cfg::Config> fmt::Debug for Addr<C> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Addr")
+            .field("addr", &format_args!("{:#0x}", &self.addr))
+            .field("index", &self.index())
+            .field("offset", &self.offset())
+            .finish()
+    }
+}
+
+impl<C: cfg::Config> PartialEq for Addr<C> {
+    fn eq(&self, other: &Self) -> bool {
+        self.addr == other.addr
+    }
+}
+
+impl<C: cfg::Config> Eq for Addr<C> {}
+
+impl<C: cfg::Config> PartialOrd for Addr<C> {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.addr.partial_cmp(&other.addr)
+    }
+}
+
+impl<C: cfg::Config> Ord for Addr<C> {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.addr.cmp(&other.addr)
+    }
+}
+
+impl<C: cfg::Config> Clone for Addr<C> {
+    fn clone(&self) -> Self {
+        Self::from_usize(self.addr)
+    }
+}
+
+impl<C: cfg::Config> Copy for Addr<C> {}
+
+#[inline(always)]
+pub(crate) fn indices<C: cfg::Config>(idx: usize) -> (Addr<C>, usize) {
+    let addr = C::unpack_addr(idx);
+    (addr, addr.index())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::Pack;
+    use proptest::prelude::*;
+
+    proptest! {
+        #[test]
+        fn addr_roundtrips(pidx in 0usize..Addr::<cfg::DefaultConfig>::BITS) {
+            let addr = Addr::<cfg::DefaultConfig>::from_usize(pidx);
+            let packed = addr.pack(0);
+            assert_eq!(addr, Addr::from_packed(packed));
+        }
+        #[test]
+        fn gen_roundtrips(gen in 0usize..slot::Generation::<cfg::DefaultConfig>::BITS) {
+            let gen = slot::Generation::<cfg::DefaultConfig>::from_usize(gen);
+            let packed = gen.pack(0);
+            assert_eq!(gen, slot::Generation::from_packed(packed));
+        }
+
+        #[test]
+        fn page_roundtrips(
+            gen in 0usize..slot::Generation::<cfg::DefaultConfig>::BITS,
+            addr in 0usize..Addr::<cfg::DefaultConfig>::BITS,
+        ) {
+            let gen = slot::Generation::<cfg::DefaultConfig>::from_usize(gen);
+            let addr = Addr::<cfg::DefaultConfig>::from_usize(addr);
+            let packed = gen.pack(addr.pack(0));
+            assert_eq!(addr, Addr::from_packed(packed));
+            assert_eq!(gen, slot::Generation::from_packed(packed));
+        }
+    }
+}
diff --git a/vendor/sharded-slab/src/page/slot.rs b/vendor/sharded-slab/src/page/slot.rs
new file mode 100644
index 000000000..3387d5388
--- /dev/null
+++ b/vendor/sharded-slab/src/page/slot.rs
@@ -0,0 +1,920 @@
+use super::FreeList;
+use crate::sync::{
+    atomic::{AtomicUsize, Ordering},
+    hint, UnsafeCell,
+};
+use crate::{cfg, clear::Clear, Pack, Tid};
+use std::{fmt, marker::PhantomData, mem, ptr, thread};
+
+pub(crate) struct Slot<T, C> {
+    lifecycle: AtomicUsize,
+    /// The offset of the next item on the free list.
+    next: UnsafeCell<usize>,
+    /// The data stored in the slot.
+    item: UnsafeCell<T>,
+    _cfg: PhantomData<fn(C)>,
+}
+
+#[derive(Debug)]
+pub(crate) struct Guard<T, C: cfg::Config = cfg::DefaultConfig> {
+    slot: ptr::NonNull<Slot<T, C>>,
+}
+
+#[derive(Debug)]
+pub(crate) struct InitGuard<T, C: cfg::Config = cfg::DefaultConfig> {
+    slot: ptr::NonNull<Slot<T, C>>,
+    curr_lifecycle: usize,
+    released: bool,
+}
+
+#[repr(transparent)]
+pub(crate) struct Generation<C = cfg::DefaultConfig> {
+    value: usize,
+    _cfg: PhantomData<fn(C)>,
+}
+
+#[repr(transparent)]
+pub(crate) struct RefCount<C = cfg::DefaultConfig> {
+    value: usize,
+    _cfg: PhantomData<fn(C)>,
+}
+
+pub(crate) struct Lifecycle<C> {
+    state: State,
+    _cfg: PhantomData<fn(C)>,
+}
+struct LifecycleGen<C>(Generation<C>);
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+#[repr(usize)]
+enum State {
+    Present = 0b00,
+    Marked = 0b01,
+    Removing = 0b11,
+}
+
+impl<C: cfg::Config> Pack<C> for Generation<C> {
+    /// Use all the remaining bits in the word for the generation counter, minus
+    /// any bits reserved by the user.
+    const LEN: usize = (cfg::WIDTH - C::RESERVED_BITS) - Self::SHIFT;
+
+    type Prev = Tid<C>;
+
+    #[inline(always)]
+    fn from_usize(u: usize) -> Self {
+        debug_assert!(u <= Self::BITS);
+        Self::new(u)
+    }
+
+    #[inline(always)]
+    fn as_usize(&self) -> usize {
+        self.value
+    }
+}
+
+impl<C: cfg::Config> Generation<C> {
+    fn new(value: usize) -> Self {
+        Self {
+            value,
+            _cfg: PhantomData,
+        }
+    }
+}
+
+// Slot methods which should work across all trait bounds
+impl<T, C> Slot<T, C>
+where
+    C: cfg::Config,
+{
+    #[inline(always)]
+    pub(super) fn next(&self) -> usize {
+        self.next.with(|next| unsafe { *next })
+    }
+
+    #[inline(always)]
+    pub(crate) fn value(&self) -> &T {
+        self.item.with(|item| unsafe { &*item })
+    }
+
+    #[inline(always)]
+    pub(super) fn set_next(&self, next: usize) {
+        self.next.with_mut(|n| unsafe {
+            (*n) = next;
+        })
+    }
+
+    #[inline(always)]
+    pub(crate) fn get(&self, gen: Generation<C>) -> Option<Guard<T, C>> {
+        let mut lifecycle = self.lifecycle.load(Ordering::Acquire);
+        loop {
+            // Unpack the current state.
+            let state = Lifecycle::<C>::from_packed(lifecycle);
+            let current_gen = LifecycleGen::<C>::from_packed(lifecycle).0;
+            let refs = RefCount::<C>::from_packed(lifecycle);
+
+            test_println!(
+                "-> get {:?}; current_gen={:?}; lifecycle={:#x}; state={:?}; refs={:?};",
+                gen,
+                current_gen,
+                lifecycle,
+                state,
+                refs,
+            );
+
+            // Is it okay to access this slot? The accessed generation must be
+            // current, and the slot must not be in the process of being
+            // removed. If we can no longer access the slot at the given
+            // generation, return `None`.
+            if gen != current_gen || state != Lifecycle::PRESENT {
+                test_println!("-> get: no longer exists!");
+                return None;
+            }
+
+            // Try to increment the slot's ref count by one.
+            let new_refs = refs.incr()?;
+            match self.lifecycle.compare_exchange(
+                lifecycle,
+                new_refs.pack(current_gen.pack(state.pack(0))),
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => {
+                    test_println!("-> {:?}", new_refs);
+                    return Some(Guard {
+                        slot: ptr::NonNull::from(self),
+                    });
+                }
+                Err(actual) => {
+                    // Another thread modified the slot's state before us! We
+                    // need to retry with the new state.
+                    //
+                    // Since the new state may mean that the accessed generation
+                    // is no longer valid, we'll check again on the next
+                    // iteration of the loop.
+                    test_println!("-> get: retrying; lifecycle={:#x};", actual);
+                    lifecycle = actual;
+                }
+            };
+        }
+    }
+
+    /// Marks this slot to be released, returning `true` if the slot can be
+    /// mutated *now* and `false` otherwise.
+    ///
+    /// This method checks if there are any references to this slot. If there _are_ valid
+    /// references, it just marks them for modification and returns and the next thread calling
+    /// either `clear_storage` or `remove_value` will try and modify the storage
+    fn mark_release(&self, gen: Generation<C>) -> Option<bool> {
+        let mut lifecycle = self.lifecycle.load(Ordering::Acquire);
+        let mut curr_gen;
+
+        // Try to advance the slot's state to "MARKED", which indicates that it
+        // should be removed when it is no longer concurrently accessed.
+        loop {
+            curr_gen = LifecycleGen::from_packed(lifecycle).0;
+            test_println!(
+                "-> mark_release; gen={:?}; current_gen={:?};",
+                gen,
+                curr_gen
+            );
+
+            // Is the slot still at the generation we are trying to remove?
+            if gen != curr_gen {
+                return None;
+            }
+
+            let state = Lifecycle::<C>::from_packed(lifecycle).state;
+            test_println!("-> mark_release; state={:?};", state);
+            match state {
+                State::Removing => {
+                    test_println!("--> mark_release; cannot release (already removed!)");
+                    return None;
+                }
+                State::Marked => {
+                    test_println!("--> mark_release; already marked;");
+                    break;
+                }
+                State::Present => {}
+            };
+
+            // Set the new state to `MARKED`.
+            let new_lifecycle = Lifecycle::<C>::MARKED.pack(lifecycle);
+            test_println!(
+                "-> mark_release; old_lifecycle={:#x}; new_lifecycle={:#x};",
+                lifecycle,
+                new_lifecycle
+            );
+
+            match self.lifecycle.compare_exchange(
+                lifecycle,
+                new_lifecycle,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => break,
+                Err(actual) => {
+                    test_println!("-> mark_release; retrying");
+                    lifecycle = actual;
+                }
+            }
+        }
+
+        // Unpack the current reference count to see if we can remove the slot now.
+        let refs = RefCount::<C>::from_packed(lifecycle);
+        test_println!("-> mark_release: marked; refs={:?};", refs);
+
+        // Are there currently outstanding references to the slot? If so, it
+        // will have to be removed when those references are dropped.
+        Some(refs.value == 0)
+    }
+
+    /// Mutates this slot.
+    ///
+    /// This method spins until no references to this slot are left, and calls the mutator
+    fn release_with<F, M, R>(&self, gen: Generation<C>, offset: usize, free: &F, mutator: M) -> R
+    where
+        F: FreeList<C>,
+        M: FnOnce(Option<&mut T>) -> R,
+    {
+        let mut lifecycle = self.lifecycle.load(Ordering::Acquire);
+        let mut advanced = false;
+        // Exponential spin backoff while waiting for the slot to be released.
+        let mut spin_exp = 0;
+        let next_gen = gen.advance();
+        loop {
+            let current_gen = Generation::from_packed(lifecycle);
+            test_println!("-> release_with; lifecycle={:#x}; expected_gen={:?}; current_gen={:?}; next_gen={:?};",
+                lifecycle,
+                gen,
+                current_gen,
+                next_gen
+            );
+
+            // First, make sure we are actually able to remove the value.
+            // If we're going to remove the value, the generation has to match
+            // the value that `remove_value` was called with...unless we've
+            // already stored the new generation.
+            if (!advanced) && gen != current_gen {
+                test_println!("-> already removed!");
+                return mutator(None);
+            }
+
+            match self.lifecycle.compare_exchange(
+                lifecycle,
+                next_gen.pack(lifecycle),
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(actual) => {
+                    // If we're in this state, we have successfully advanced to
+                    // the next generation.
+                    advanced = true;
+
+                    // Make sure that there are no outstanding references.
+                    let refs = RefCount::<C>::from_packed(actual);
+                    test_println!("-> advanced gen; lifecycle={:#x}; refs={:?};", actual, refs);
+                    if refs.value == 0 {
+                        test_println!("-> ok to remove!");
+                        // safety: we've modified the generation of this slot and any other thread
+                        // calling this method will exit out at the generation check above in the
+                        // next iteraton of the loop.
+                        let value = self
+                            .item
+                            .with_mut(|item| mutator(Some(unsafe { &mut *item })));
+                        free.push(offset, self);
+                        return value;
+                    }
+
+                    // Otherwise, a reference must be dropped before we can
+                    // remove the value. Spin here until there are no refs remaining...
+                    test_println!("-> refs={:?}; spin...", refs);
+
+                    // Back off, spinning and possibly yielding.
+                    exponential_backoff(&mut spin_exp);
+                }
+                Err(actual) => {
+                    test_println!("-> retrying; lifecycle={:#x};", actual);
+                    lifecycle = actual;
+                    // The state changed; reset the spin backoff.
+                    spin_exp = 0;
+                }
+            }
+        }
+    }
+
+    /// Initialize a slot
+    ///
+    /// This method initializes and sets up the state for a slot. When being used in `Pool`, we
+    /// only need to ensure that the `Slot` is in the right `state, while when being used in a
+    /// `Slab` we want to insert a value into it, as the memory is not initialized
+    pub(crate) fn init(&self) -> Option<InitGuard<T, C>> {
+        // Load the current lifecycle state.
+        let lifecycle = self.lifecycle.load(Ordering::Acquire);
+        let gen = LifecycleGen::<C>::from_packed(lifecycle).0;
+        let refs = RefCount::<C>::from_packed(lifecycle);
+
+        test_println!(
+            "-> initialize_state; state={:?}; gen={:?}; refs={:?};",
+            Lifecycle::<C>::from_packed(lifecycle),
+            gen,
+            refs,
+        );
+
+        if refs.value != 0 {
+            test_println!("-> initialize while referenced! cancelling");
+            return None;
+        }
+
+        Some(InitGuard {
+            slot: ptr::NonNull::from(self),
+            curr_lifecycle: lifecycle,
+            released: false,
+        })
+    }
+}
+
+// Slot impl which _needs_ an `Option` for self.item, this is for `Slab` to use.
+impl<T, C> Slot<Option<T>, C>
+where
+    C: cfg::Config,
+{
+    fn is_empty(&self) -> bool {
+        self.item.with(|item| unsafe { (*item).is_none() })
+    }
+
+    /// Insert a value into a slot
+    ///
+    /// We first initialize the state and then insert the pased in value into the slot.
+    #[inline]
+    pub(crate) fn insert(&self, value: &mut Option<T>) -> Option<Generation<C>> {
+        debug_assert!(self.is_empty(), "inserted into full slot");
+        debug_assert!(value.is_some(), "inserted twice");
+
+        let mut guard = self.init()?;
+        let gen = guard.generation();
+        unsafe {
+            // Safety: Accessing the value of an `InitGuard` is unsafe because
+            // it has a pointer to a slot which may dangle. Here, we know the
+            // pointed slot is alive because we have a reference to it in scope,
+            // and the `InitGuard` will be dropped when this function returns.
+            mem::swap(guard.value_mut(), value);
+            guard.release();
+        };
+        test_println!("-> inserted at {:?}", gen);
+
+        Some(gen)
+    }
+
+    /// Tries to remove the value in the slot, returning `true` if the value was
+    /// removed.
+    ///
+    /// This method tries to remove the value in the slot. If there are existing references, then
+    /// the slot is marked for removal and the next thread calling either this method or
+    /// `remove_value` will do the work instead.
+    #[inline]
+    pub(super) fn try_remove_value<F: FreeList<C>>(
+        &self,
+        gen: Generation<C>,
+        offset: usize,
+        free: &F,
+    ) -> bool {
+        let should_remove = match self.mark_release(gen) {
+            // If `mark_release` returns `Some`, a value exists at this
+            // generation. The bool inside this option indicates whether or not
+            // _we're_ allowed to remove the value.
+            Some(should_remove) => should_remove,
+            // Otherwise, the generation we tried to remove has already expired,
+            // and we did not mark anything for removal.
+            None => {
+                test_println!(
+                    "-> try_remove_value; nothing exists at generation={:?}",
+                    gen
+                );
+                return false;
+            }
+        };
+
+        test_println!("-> try_remove_value; marked!");
+
+        if should_remove {
+            // We're allowed to remove the slot now!
+            test_println!("-> try_remove_value; can remove now");
+            self.remove_value(gen, offset, free);
+        }
+
+        true
+    }
+
+    #[inline]
+    pub(super) fn remove_value<F: FreeList<C>>(
+        &self,
+        gen: Generation<C>,
+        offset: usize,
+        free: &F,
+    ) -> Option<T> {
+        self.release_with(gen, offset, free, |item| item.and_then(Option::take))
+    }
+}
+
+// These impls are specific to `Pool`
+impl<T, C> Slot<T, C>
+where
+    T: Default + Clear,
+    C: cfg::Config,
+{
+    pub(in crate::page) fn new(next: usize) -> Self {
+        Self {
+            lifecycle: AtomicUsize::new(Lifecycle::<C>::REMOVING.as_usize()),
+            item: UnsafeCell::new(T::default()),
+            next: UnsafeCell::new(next),
+            _cfg: PhantomData,
+        }
+    }
+
+    /// Try to clear this slot's storage
+    ///
+    /// If there are references to this slot, then we mark this slot for clearing and let the last
+    /// thread do the work for us.
+    #[inline]
+    pub(super) fn try_clear_storage<F: FreeList<C>>(
+        &self,
+        gen: Generation<C>,
+        offset: usize,
+        free: &F,
+    ) -> bool {
+        let should_clear = match self.mark_release(gen) {
+            // If `mark_release` returns `Some`, a value exists at this
+            // generation. The bool inside this option indicates whether or not
+            // _we're_ allowed to clear the value.
+            Some(should_clear) => should_clear,
+            // Otherwise, the generation we tried to remove has already expired,
+            // and we did not mark anything for removal.
+            None => {
+                test_println!(
+                    "-> try_clear_storage; nothing exists at generation={:?}",
+                    gen
+                );
+                return false;
+            }
+        };
+
+        test_println!("-> try_clear_storage; marked!");
+
+        if should_clear {
+            // We're allowed to remove the slot now!
+            test_println!("-> try_remove_value; can clear now");
+            return self.clear_storage(gen, offset, free);
+        }
+
+        true
+    }
+
+    /// Clear this slot's storage
+    ///
+    /// This method blocks until all references have been dropped and clears the storage.
+    pub(super) fn clear_storage<F: FreeList<C>>(
+        &self,
+        gen: Generation<C>,
+        offset: usize,
+        free: &F,
+    ) -> bool {
+        // release_with will _always_ wait unitl it can release the slot or just return if the slot
+        // has already been released.
+        self.release_with(gen, offset, free, |item| {
+            let cleared = item.map(|inner| Clear::clear(inner)).is_some();
+            test_println!("-> cleared: {}", cleared);
+            cleared
+        })
+    }
+}
+
+impl<T, C: cfg::Config> Slot<T, C> {
+    fn release(&self) -> bool {
+        let mut lifecycle = self.lifecycle.load(Ordering::Acquire);
+        loop {
+            let refs = RefCount::<C>::from_packed(lifecycle);
+            let state = Lifecycle::<C>::from_packed(lifecycle).state;
+            let gen = LifecycleGen::<C>::from_packed(lifecycle).0;
+
+            // Are we the last guard, and is the slot marked for removal?
+            let dropping = refs.value == 1 && state == State::Marked;
+            let new_lifecycle = if dropping {
+                // If so, we want to advance the state to "removing"
+                gen.pack(State::Removing as usize)
+            } else {
+                // Otherwise, just subtract 1 from the ref count.
+                refs.decr().pack(lifecycle)
+            };
+
+            test_println!(
+                "-> drop guard: state={:?}; gen={:?}; refs={:?}; lifecycle={:#x}; new_lifecycle={:#x}; dropping={:?}",
+                state,
+                gen,
+                refs,
+                lifecycle,
+                new_lifecycle,
+                dropping
+            );
+            match self.lifecycle.compare_exchange(
+                lifecycle,
+                new_lifecycle,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => {
+                    test_println!("-> drop guard: done;  dropping={:?}", dropping);
+                    return dropping;
+                }
+                Err(actual) => {
+                    test_println!("-> drop guard; retry, actual={:#x}", actual);
+                    lifecycle = actual;
+                }
+            }
+        }
+    }
+}
+
+impl<T, C: cfg::Config> fmt::Debug for Slot<T, C> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let lifecycle = self.lifecycle.load(Ordering::Relaxed);
+        f.debug_struct("Slot")
+            .field("lifecycle", &format_args!("{:#x}", lifecycle))
+            .field("state", &Lifecycle::<C>::from_packed(lifecycle).state)
+            .field("gen", &LifecycleGen::<C>::from_packed(lifecycle).0)
+            .field("refs", &RefCount::<C>::from_packed(lifecycle))
+            .field("next", &self.next())
+            .finish()
+    }
+}
+
+// === impl Generation ===
+
+impl<C> fmt::Debug for Generation<C> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Generation").field(&self.value).finish()
+    }
+}
+
+impl<C: cfg::Config> Generation<C> {
+    fn advance(self) -> Self {
+        Self::from_usize((self.value + 1) % Self::BITS)
+    }
+}
+
+impl<C: cfg::Config> PartialEq for Generation<C> {
+    fn eq(&self, other: &Self) -> bool {
+        self.value == other.value
+    }
+}
+
+impl<C: cfg::Config> Eq for Generation<C> {}
+
+impl<C: cfg::Config> PartialOrd for Generation<C> {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.value.partial_cmp(&other.value)
+    }
+}
+
+impl<C: cfg::Config> Ord for Generation<C> {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.value.cmp(&other.value)
+    }
+}
+
+impl<C: cfg::Config> Clone for Generation<C> {
+    fn clone(&self) -> Self {
+        Self::new(self.value)
+    }
+}
+
+impl<C: cfg::Config> Copy for Generation<C> {}
+
+// === impl Guard ===
+
+impl<T, C: cfg::Config> Guard<T, C> {
+    /// Releases the guard, returning `true` if the slot should be cleared.
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `Guard` does not outlive the slab that contains
+    /// the pointed slot. Failure to do so means this pointer may dangle.
+    #[inline]
+    pub(crate) unsafe fn release(&self) -> bool {
+        self.slot().release()
+    }
+
+    /// Returns a borrowed reference to the slot.
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `Guard` does not outlive the slab that contains
+    /// the pointed slot. Failure to do so means this pointer may dangle.
+    #[inline]
+    pub(crate) unsafe fn slot(&self) -> &Slot<T, C> {
+        self.slot.as_ref()
+    }
+
+    /// Returns a borrowed reference to the slot's value.
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `Guard` does not outlive the slab that contains
+    /// the pointed slot. Failure to do so means this pointer may dangle.
+    #[inline(always)]
+    pub(crate) unsafe fn value(&self) -> &T {
+        self.slot().item.with(|item| &*item)
+    }
+}
+
+// === impl Lifecycle ===
+
+impl<C: cfg::Config> Lifecycle<C> {
+    const MARKED: Self = Self {
+        state: State::Marked,
+        _cfg: PhantomData,
+    };
+    const REMOVING: Self = Self {
+        state: State::Removing,
+        _cfg: PhantomData,
+    };
+    const PRESENT: Self = Self {
+        state: State::Present,
+        _cfg: PhantomData,
+    };
+}
+
+impl<C: cfg::Config> Pack<C> for Lifecycle<C> {
+    const LEN: usize = 2;
+    type Prev = ();
+
+    fn from_usize(u: usize) -> Self {
+        Self {
+            state: match u & Self::MASK {
+                0b00 => State::Present,
+                0b01 => State::Marked,
+                0b11 => State::Removing,
+                bad => unreachable!("weird lifecycle {:#b}", bad),
+            },
+            _cfg: PhantomData,
+        }
+    }
+
+    fn as_usize(&self) -> usize {
+        self.state as usize
+    }
+}
+
+impl<C> PartialEq for Lifecycle<C> {
+    fn eq(&self, other: &Self) -> bool {
+        self.state == other.state
+    }
+}
+
+impl<C> Eq for Lifecycle<C> {}
+
+impl<C> fmt::Debug for Lifecycle<C> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Lifecycle").field(&self.state).finish()
+    }
+}
+
+// === impl RefCount ===
+
+impl<C: cfg::Config> Pack<C> for RefCount<C> {
+    const LEN: usize = cfg::WIDTH - (Lifecycle::<C>::LEN + Generation::<C>::LEN);
+    type Prev = Lifecycle<C>;
+
+    fn from_usize(value: usize) -> Self {
+        debug_assert!(value <= Self::BITS);
+        Self {
+            value,
+            _cfg: PhantomData,
+        }
+    }
+
+    fn as_usize(&self) -> usize {
+        self.value
+    }
+}
+
+impl<C: cfg::Config> RefCount<C> {
+    pub(crate) const MAX: usize = Self::BITS - 1;
+
+    #[inline]
+    fn incr(self) -> Option<Self> {
+        if self.value >= Self::MAX {
+            test_println!("-> get: {}; MAX={}", self.value, RefCount::<C>::MAX);
+            return None;
+        }
+
+        Some(Self::from_usize(self.value + 1))
+    }
+
+    #[inline]
+    fn decr(self) -> Self {
+        Self::from_usize(self.value - 1)
+    }
+}
+
+impl<C> fmt::Debug for RefCount<C> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("RefCount").field(&self.value).finish()
+    }
+}
+
+impl<C: cfg::Config> PartialEq for RefCount<C> {
+    fn eq(&self, other: &Self) -> bool {
+        self.value == other.value
+    }
+}
+
+impl<C: cfg::Config> Eq for RefCount<C> {}
+
+impl<C: cfg::Config> PartialOrd for RefCount<C> {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.value.partial_cmp(&other.value)
+    }
+}
+
+impl<C: cfg::Config> Ord for RefCount<C> {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.value.cmp(&other.value)
+    }
+}
+
+impl<C: cfg::Config> Clone for RefCount<C> {
+    fn clone(&self) -> Self {
+        Self::from_usize(self.value)
+    }
+}
+
+impl<C: cfg::Config> Copy for RefCount<C> {}
+
+// === impl LifecycleGen ===
+
+impl<C: cfg::Config> Pack<C> for LifecycleGen<C> {
+    const LEN: usize = Generation::<C>::LEN;
+    type Prev = RefCount<C>;
+
+    fn from_usize(value: usize) -> Self {
+        Self(Generation::from_usize(value))
+    }
+
+    fn as_usize(&self) -> usize {
+        self.0.as_usize()
+    }
+}
+
+impl<T, C: cfg::Config> InitGuard<T, C> {
+    pub(crate) fn generation(&self) -> Generation<C> {
+        LifecycleGen::<C>::from_packed(self.curr_lifecycle).0
+    }
+
+    /// Returns a borrowed reference to the slot's value.
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `InitGuard` does not outlive the slab that
+    /// contains the pointed slot. Failure to do so means this pointer may
+    /// dangle.
+    pub(crate) unsafe fn value(&self) -> &T {
+        self.slot.as_ref().item.with(|val| &*val)
+    }
+
+    /// Returns a mutably borrowed reference to the slot's value.
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `InitGuard` does not outlive the slab that
+    /// contains the pointed slot. Failure to do so means this pointer may
+    /// dangle.
+    ///
+    /// It's safe to reference the slot mutably, though, because creating an
+    /// `InitGuard` ensures there are no outstanding immutable references.
+    pub(crate) unsafe fn value_mut(&mut self) -> &mut T {
+        self.slot.as_ref().item.with_mut(|val| &mut *val)
+    }
+
+    /// Releases the guard, returning `true` if the slot should be cleared.
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `InitGuard` does not outlive the slab that
+    /// contains the pointed slot. Failure to do so means this pointer may
+    /// dangle.
+    pub(crate) unsafe fn release(&mut self) -> bool {
+        self.release2(0)
+    }
+
+    /// Downgrades the guard to an immutable guard
+    ///
+    /// ## Safety
+    ///
+    /// This dereferences a raw pointer to the slot. The caller is responsible
+    /// for ensuring that the `InitGuard` does not outlive the slab that
+    /// contains the pointed slot. Failure to do so means this pointer may
+    /// dangle.
+    pub(crate) unsafe fn downgrade(&mut self) -> Guard<T, C> {
+        let _ = self.release2(RefCount::<C>::from_usize(1).pack(0));
+        Guard { slot: self.slot }
+    }
+
+    unsafe fn release2(&mut self, new_refs: usize) -> bool {
+        test_println!(
+            "InitGuard::release; curr_lifecycle={:?}; downgrading={}",
+            Lifecycle::<C>::from_packed(self.curr_lifecycle),
+            new_refs != 0,
+        );
+        if self.released {
+            test_println!("-> already released!");
+            return false;
+        }
+        self.released = true;
+        let mut curr_lifecycle = self.curr_lifecycle;
+        let slot = self.slot.as_ref();
+        let new_lifecycle = LifecycleGen::<C>::from_packed(self.curr_lifecycle)
+            .pack(Lifecycle::<C>::PRESENT.pack(new_refs));
+
+        match slot.lifecycle.compare_exchange(
+            curr_lifecycle,
+            new_lifecycle,
+            Ordering::AcqRel,
+            Ordering::Acquire,
+        ) {
+            Ok(_) => {
+                test_println!("--> advanced to PRESENT; done");
+                return false;
+            }
+            Err(actual) => {
+                test_println!(
+                    "--> lifecycle changed; actual={:?}",
+                    Lifecycle::<C>::from_packed(actual)
+                );
+                curr_lifecycle = actual;
+            }
+        }
+
+        // if the state was no longer the prior state, we are now responsible
+        // for releasing the slot.
+        loop {
+            let refs = RefCount::<C>::from_packed(curr_lifecycle);
+            let state = Lifecycle::<C>::from_packed(curr_lifecycle).state;
+
+            test_println!(
+                "-> InitGuard::release; lifecycle={:#x}; state={:?}; refs={:?};",
+                curr_lifecycle,
+                state,
+                refs,
+            );
+
+            debug_assert!(state == State::Marked || thread::panicking(), "state was not MARKED; someone else has removed the slot while we have exclusive access!\nactual={:?}", state);
+            debug_assert!(refs.value == 0 || thread::panicking(), "ref count was not 0; someone else has referenced the slot while we have exclusive access!\nactual={:?}", refs);
+            let new_lifecycle = self.generation().pack(State::Removing as usize);
+
+            match slot.lifecycle.compare_exchange(
+                curr_lifecycle,
+                new_lifecycle,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => {
+                    test_println!("-> InitGuard::RELEASE: done!");
+                    return true;
+                }
+                Err(actual) => {
+                    debug_assert!(thread::panicking(), "we should not have to retry this CAS!");
+                    test_println!("-> InitGuard::release; retry, actual={:#x}", actual);
+                    curr_lifecycle = actual;
+                }
+            }
+        }
+    }
+}
+
+// === helpers ===
+
+#[inline(always)]
+fn exponential_backoff(exp: &mut usize) {
+    /// Maximum exponent we can back off to.
+    const MAX_EXPONENT: usize = 8;
+
+    // Issue 2^exp pause instructions.
+    for _ in 0..(1 << *exp) {
+        hint::spin_loop();
+    }
+
+    if *exp >= MAX_EXPONENT {
+        // If we have reached the max backoff, also yield to the scheduler
+        // explicitly.
+        crate::sync::yield_now();
+    } else {
+        // Otherwise, increment the exponent.
+        *exp += 1;
+    }
+}
diff --git a/vendor/sharded-slab/src/page/stack.rs b/vendor/sharded-slab/src/page/stack.rs
new file mode 100644
index 000000000..e28d9b1a7
--- /dev/null
+++ b/vendor/sharded-slab/src/page/stack.rs
@@ -0,0 +1,124 @@
+use crate::cfg;
+use crate::sync::atomic::{AtomicUsize, Ordering};
+use std::{fmt, marker::PhantomData};
+
+pub(super) struct TransferStack<C = cfg::DefaultConfig> {
+    head: AtomicUsize,
+    _cfg: PhantomData<fn(C)>,
+}
+
+impl<C: cfg::Config> TransferStack<C> {
+    pub(super) fn new() -> Self {
+        Self {
+            head: AtomicUsize::new(super::Addr::<C>::NULL),
+            _cfg: PhantomData,
+        }
+    }
+
+    pub(super) fn pop_all(&self) -> Option<usize> {
+        let val = self.head.swap(super::Addr::<C>::NULL, Ordering::Acquire);
+        test_println!("-> pop {:#x}", val);
+        if val == super::Addr::<C>::NULL {
+            None
+        } else {
+            Some(val)
+        }
+    }
+
+    fn push(&self, new_head: usize, before: impl Fn(usize)) {
+        // We loop to win the race to set the new head. The `next` variable
+        // is the next slot on the stack which needs to be pointed to by the
+        // new head.
+        let mut next = self.head.load(Ordering::Relaxed);
+        loop {
+            test_println!("-> next {:#x}", next);
+            before(next);
+
+            match self
+                .head
+                .compare_exchange(next, new_head, Ordering::Release, Ordering::Relaxed)
+            {
+                // lost the race!
+                Err(actual) => {
+                    test_println!("-> retry!");
+                    next = actual;
+                }
+                Ok(_) => {
+                    test_println!("-> successful; next={:#x}", next);
+                    return;
+                }
+            }
+        }
+    }
+}
+
+impl<C: cfg::Config> super::FreeList<C> for TransferStack<C> {
+    fn push<T>(&self, new_head: usize, slot: &super::Slot<T, C>) {
+        self.push(new_head, |next| slot.set_next(next))
+    }
+}
+
+impl<C> fmt::Debug for TransferStack<C> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TransferStack")
+            .field(
+                "head",
+                &format_args!("{:#0x}", &self.head.load(Ordering::Relaxed)),
+            )
+            .finish()
+    }
+}
+
+#[cfg(all(loom, test))]
+mod test {
+    use super::*;
+    use crate::{sync::UnsafeCell, test_util};
+    use loom::thread;
+    use std::sync::Arc;
+
+    #[test]
+    fn transfer_stack() {
+        test_util::run_model("transfer_stack", || {
+            let causalities = [UnsafeCell::new(999), UnsafeCell::new(999)];
+            let shared = Arc::new((causalities, TransferStack::<cfg::DefaultConfig>::new()));
+            let shared1 = shared.clone();
+            let shared2 = shared.clone();
+
+            let t1 = thread::spawn(move || {
+                let (causalities, stack) = &*shared1;
+                stack.push(0, |prev| {
+                    causalities[0].with_mut(|c| unsafe {
+                        *c = 0;
+                    });
+                    test_println!("prev={:#x}", prev)
+                });
+            });
+            let t2 = thread::spawn(move || {
+                let (causalities, stack) = &*shared2;
+                stack.push(1, |prev| {
+                    causalities[1].with_mut(|c| unsafe {
+                        *c = 1;
+                    });
+                    test_println!("prev={:#x}", prev)
+                });
+            });
+
+            let (causalities, stack) = &*shared;
+            let mut idx = stack.pop_all();
+            while idx == None {
+                idx = stack.pop_all();
+                thread::yield_now();
+            }
+            let idx = idx.unwrap();
+            causalities[idx].with(|val| unsafe {
+                assert_eq!(
+                    *val, idx,
+                    "UnsafeCell write must happen-before index is pushed to the stack!"
+                );
+            });
+
+            t1.join().unwrap();
+            t2.join().unwrap();
+        });
+    }
+}