epoch: Fix stacked borrows violations

ci/miri.sh

@@ -16,8 +16,7 @@ MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disab
     -p crossbeam-utils \
     -p crossbeam-channel 2>&1 | ts -i '%.s  '
 
-# -Zmiri-disable-stacked-borrows is needed for https://github.com/crossbeam-rs/crossbeam/issues/545
-MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-isolation -Zmiri-disable-stacked-borrows" \
+MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-isolation" \
     cargo miri test \
     -p crossbeam-epoch 2>&1 | ts -i '%.s  '
 
@@ -27,15 +26,13 @@ MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disab
     cargo miri test \
     -p crossbeam-skiplist 2>&1 | ts -i '%.s  '
 
-# -Zmiri-disable-stacked-borrows is needed for https://github.com/crossbeam-rs/crossbeam/issues/545
 # -Zmiri-compare-exchange-weak-failure-rate=0.0 is needed because some sequential tests (e.g.,
 # doctest of Stealer::steal) incorrectly assume that sequential weak CAS will never fail.
 # -Zmiri-preemption-rate=0 is needed because this code technically has UB and Miri catches that.
-MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-compare-exchange-weak-failure-rate=0.0 -Zmiri-preemption-rate=0" \
+MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-compare-exchange-weak-failure-rate=0.0 -Zmiri-preemption-rate=0" \
     cargo miri test \
     -p crossbeam-deque 2>&1 | ts -i '%.s  '
 
-# -Zmiri-disable-stacked-borrows is needed for https://github.com/crossbeam-rs/crossbeam/issues/545
-MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-stacked-borrows" \
+MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-symbolic-alignment-check" \
     cargo miri test \
     -p crossbeam 2>&1 | ts -i '%.s  '

crossbeam-epoch/src/atomic.rs

@@ -184,7 +184,7 @@ pub trait Pointable {
     /// - The given `ptr` should have been initialized with [`Pointable::init`].
     /// - `ptr` should not have yet been dropped by [`Pointable::drop`].
     /// - `ptr` should not be mutably dereferenced by [`Pointable::deref_mut`] concurrently.
-    unsafe fn deref<'a>(ptr: *mut ()) -> &'a Self;
+    unsafe fn as_ptr(ptr: *mut ()) -> *const Self;
 
     /// Mutably dereferences the given pointer.
     ///
@@ -194,7 +194,7 @@ pub trait Pointable {
     /// - `ptr` should not have yet been dropped by [`Pointable::drop`].
     /// - `ptr` should not be dereferenced by [`Pointable::deref`] or [`Pointable::deref_mut`]
     ///   concurrently.
-    unsafe fn deref_mut<'a>(ptr: *mut ()) -> &'a mut Self;
+    unsafe fn as_mut_ptr(ptr: *mut ()) -> *mut Self;
 
     /// Drops the object pointed to by the given pointer.
     ///
@@ -216,12 +216,12 @@ impl<T> Pointable for T {
         Box::into_raw(Box::new(init)) as *mut ()
     }
 
-    unsafe fn deref<'a>(ptr: *mut ()) -> &'a Self {
-        &*(ptr as *const T)
+    unsafe fn as_ptr(ptr: *mut ()) -> *const Self {
+        ptr as *const T
     }
 
-    unsafe fn deref_mut<'a>(ptr: *mut ()) -> &'a mut Self {
-        &mut *(ptr as *mut T)
+    unsafe fn as_mut_ptr(ptr: *mut ()) -> *mut Self {
+        ptr as *mut T
     }
 
     unsafe fn drop(ptr: *mut ()) {
@@ -276,18 +276,20 @@ impl<T> Pointable for [MaybeUninit<T>] {
         ptr as *mut ()
     }
 
-    unsafe fn deref<'a>(ptr: *mut ()) -> &'a Self {
-        let array = &*(ptr as *const Array<T>);
-        slice::from_raw_parts(array.elements.as_ptr() as *const _, array.len)
+    unsafe fn as_ptr(ptr: *mut ()) -> *const Self {
+        let array = ptr as *mut Array<T>;
+        // deallocation requires raw/mut provenance: <https://github.com/rust-lang/rust/pull/67339>
+        let data = ptr::addr_of_mut!((*array).elements);
+        slice::from_raw_parts(data as *const _, (*array).len)
     }
 
-    unsafe fn deref_mut<'a>(ptr: *mut ()) -> &'a mut Self {
-        let array = &*(ptr as *mut Array<T>);
-        slice::from_raw_parts_mut(array.elements.as_ptr() as *mut _, array.len)
+    unsafe fn as_mut_ptr(ptr: *mut ()) -> *mut Self {
+        let array = &mut *(ptr as *mut Array<T>);
+        slice::from_raw_parts_mut(array.elements.as_mut_ptr(), array.len)
     }
 
     unsafe fn drop(ptr: *mut ()) {
-        let array = &*(ptr as *mut Array<T>);
+        let array = &mut *(ptr as *mut Array<T>);
         let size = mem::size_of::<Array<T>>() + mem::size_of::<MaybeUninit<T>>() * array.len;
         let align = mem::align_of::<Array<T>>();
         let layout = alloc::Layout::from_size_align(size, align).unwrap();
@@ -1001,7 +1003,7 @@ impl<T: ?Sized + Pointable> fmt::Pointer for Atomic<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let data = self.data.load(Ordering::SeqCst);
         let (raw, _) = decompose_tag::<T>(data);
-        fmt::Pointer::fmt(&(unsafe { T::deref(raw) as *const _ }), f)
+        fmt::Pointer::fmt(&(unsafe { T::as_ptr(raw) }), f)
     }
 }
 
@@ -1287,14 +1289,14 @@ impl<T: ?Sized + Pointable> Deref for Owned<T> {
 
     fn deref(&self) -> &T {
         let (raw, _) = decompose_tag::<T>(self.data);
-        unsafe { T::deref(raw) }
+        unsafe { &*T::as_ptr(raw) }
     }
 }
 
 impl<T: ?Sized + Pointable> DerefMut for Owned<T> {
     fn deref_mut(&mut self) -> &mut T {
         let (raw, _) = decompose_tag::<T>(self.data);
-        unsafe { T::deref_mut(raw) }
+        unsafe { &mut *T::as_mut_ptr(raw) }
     }
 }
 
@@ -1480,7 +1482,7 @@ impl<'g, T: ?Sized + Pointable> Shared<'g, T> {
     /// ```
     pub unsafe fn deref(&self) -> &'g T {
         let (raw, _) = decompose_tag::<T>(self.data);
-        T::deref(raw)
+        &*T::as_ptr(raw)
     }
 
     /// Dereferences the pointer.
@@ -1522,7 +1524,7 @@ impl<'g, T: ?Sized + Pointable> Shared<'g, T> {
     /// ```
     pub unsafe fn deref_mut(&mut self) -> &'g mut T {
         let (raw, _) = decompose_tag::<T>(self.data);
-        T::deref_mut(raw)
+        &mut *T::as_mut_ptr(raw)
     }
 
     /// Converts the pointer to a reference.
@@ -1562,10 +1564,24 @@ impl<'g, T: ?Sized + Pointable> Shared<'g, T> {
         if raw.is_null() {
             None
         } else {
-            Some(T::deref(raw))
+            Some(&*T::as_ptr(raw))
         }
     }
 
+    pub(crate) unsafe fn as_non_null(&self) -> Option<*const T> {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        if raw.is_null() {
+            None
+        } else {
+            Some(T::as_ptr(raw))
+        }
+    }
+
+    pub(crate) unsafe fn as_ptr(&self) -> *const T {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        T::as_ptr(raw)
+    }
+
     /// Takes ownership of the pointee.
     ///
     /// # Panics

crossbeam-epoch/src/internal.rs

@@ -609,22 +609,18 @@ impl Local {
 }
 
 impl IsElement<Local> for Local {
-    fn entry_of(local: &Local) -> &Entry {
+    fn entry_of(local: *const Local) -> *const Entry {
         unsafe {
-            let entry_ptr =
-                (local as *const Local as *const u8).add(offset_of!(Local, entry)) as *const Entry;
-            &*entry_ptr
+            (local as *const Local as *const u8).add(offset_of!(Local, entry)) as *const Entry
         }
     }
 
-    unsafe fn element_of(entry: &Entry) -> &Local {
-        let local_ptr =
-            (entry as *const Entry as *const u8).sub(offset_of!(Local, entry)) as *const Local;
-        &*local_ptr
+    unsafe fn element_of(entry: *const Entry) -> *const Local {
+        (entry as *const Entry as *const u8).sub(offset_of!(Local, entry)) as *const Local
     }
 
-    unsafe fn finalize(entry: &Entry, guard: &Guard) {
-        guard.defer_destroy(Shared::from(Self::element_of(entry) as *const _));
+    unsafe fn finalize(entry: *const Entry, guard: &Guard) {
+        guard.defer_destroy(Shared::from(Self::element_of(entry)));
     }
 }
 

crossbeam-epoch/src/sync/list.rs

@@ -66,7 +66,7 @@ pub(crate) struct Entry {
 ///
 pub(crate) trait IsElement<T> {
     /// Returns a reference to this element's `Entry`.
-    fn entry_of(_: &T) -> &Entry;
+    fn entry_of(_: *const T) -> *const Entry;
 
     /// Given a reference to an element's entry, returns that element.
     ///
@@ -80,15 +80,15 @@ pub(crate) trait IsElement<T> {
     ///
     /// The caller has to guarantee that the `Entry` is called with was retrieved from an instance
     /// of the element type (`T`).
-    unsafe fn element_of(_: &Entry) -> &T;
+    unsafe fn element_of(_: *const Entry) -> *const T;
 
     /// The function that is called when an entry is unlinked from list.
     ///
     /// # Safety
     ///
     /// The caller has to guarantee that the `Entry` is called with was retrieved from an instance
     /// of the element type (`T`).
-    unsafe fn finalize(_: &Entry, _: &Guard);
+    unsafe fn finalize(_: *const Entry, _: &Guard);
 }
 
 /// A lock-free, intrusive linked list of type `T`.
@@ -173,16 +173,16 @@ impl<T, C: IsElement<T>> List<T, C> {
         // Insert right after head, i.e. at the beginning of the list.
         let to = &self.head;
         // Get the intrusively stored Entry of the new element to insert.
-        let entry: &Entry = C::entry_of(container.deref());
+        let entry: *const Entry = C::entry_of(container.as_ptr());
         // Make a Shared ptr to that Entry.
-        let entry_ptr = Shared::from(entry as *const _);
+        let entry_ptr = Shared::from(entry);
         // Read the current successor of where we want to insert.
         let mut next = to.load(Relaxed, guard);
 
         loop {
             // Set the Entry of the to-be-inserted element to point to the previous successor of
             // `to`.
-            entry.next.store(next, Relaxed);
+            (*entry).next.store(next, Relaxed);
             match to.compare_exchange_weak(next, entry_ptr, Release, Relaxed, guard) {
                 Ok(_) => break,
                 // We lost the race or weak CAS failed spuriously. Update the successor and try
@@ -225,7 +225,7 @@ impl<T, C: IsElement<T>> Drop for List<T, C> {
                 // Verify that all elements have been removed from the list.
                 assert_eq!(succ.tag(), 1);
 
-                C::finalize(curr.deref(), guard);
+                C::finalize(curr.as_ptr(), guard);
                 curr = succ;
             }
         }
@@ -236,8 +236,8 @@ impl<'g, T: 'g, C: IsElement<T>> Iterator for Iter<'g, T, C> {
     type Item = Result<&'g T, IterError>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        while let Some(c) = unsafe { self.curr.as_ref() } {
-            let succ = c.next.load(Acquire, self.guard);
+        while let Some(c) = unsafe { self.curr.as_non_null() } {
+            let succ = unsafe { (*c).next.load(Acquire, self.guard) };
 
             if succ.tag() == 1 {
                 // This entry was removed. Try unlinking it from the list.
@@ -257,7 +257,7 @@ impl<'g, T: 'g, C: IsElement<T>> Iterator for Iter<'g, T, C> {
                         // deallocation. Deferred drop is okay, because `list.delete()` can only be
                         // called if `T: 'static`.
                         unsafe {
-                            C::finalize(self.curr.deref(), self.guard);
+                            C::finalize(self.curr.as_ptr(), self.guard);
                         }
 
                         // `succ` is the new value of `self.pred`.
@@ -284,10 +284,10 @@ impl<'g, T: 'g, C: IsElement<T>> Iterator for Iter<'g, T, C> {
             }
 
             // Move one step forward.
-            self.pred = &c.next;
+            self.pred = unsafe { &(*c).next };
             self.curr = succ;
 
-            return Some(Ok(unsafe { C::element_of(c) }));
+            return Some(Ok(unsafe { &*C::element_of(c) }));
         }
 
         // We reached the end of the list.
@@ -303,16 +303,16 @@ mod tests {
     use std::sync::Barrier;
 
     impl IsElement<Entry> for Entry {
-        fn entry_of(entry: &Entry) -> &Entry {
+        fn entry_of(entry: *const Entry) -> *const Entry {
             entry
         }
 
-        unsafe fn element_of(entry: &Entry) -> &Entry {
+        unsafe fn element_of(entry: *const Entry) -> *const Entry {
             entry
         }
 
-        unsafe fn finalize(entry: &Entry, guard: &Guard) {
-            guard.defer_destroy(Shared::from(Self::element_of(entry) as *const _));
+        unsafe fn finalize(entry: *const Entry, guard: &Guard) {
+            guard.defer_destroy(Shared::from(Self::element_of(entry)));
         }
     }