Merge pull request #259 from moka-rs/fix-deq-mutating-through-shared-ref

Fix the caches mutating a deque node through a `NonNull` pointer derived from a shared reference

CHANGELOG.md

@@ -13,6 +13,11 @@
     - Like the `invalidate` method, this method discards any cached value for the
       key, but returns a clone of the value.
 
+### Fixed
+
+- Fixed the caches mutating a deque node through a `NonNull` pointer derived from a
+  shared reference. ([#259][gh-pull-0259])
+
 ### Removed
 
 - Removed `unsync` cache that was marked as deprecated in [v0.10.0](#version-0100).
@@ -642,6 +647,7 @@ The minimum supported Rust version (MSRV) is now 1.51.0 (2021-03-25).
 [gh-issue-0034]: https://github.com/moka-rs/moka/issues/34/
 [gh-issue-0031]: https://github.com/moka-rs/moka/issues/31/
 
+[gh-pull-0259]: https://github.com/moka-rs/moka/pull/259/
 [gh-pull-0251]: https://github.com/moka-rs/moka/pull/251/
 [gh-pull-0248]: https://github.com/moka-rs/moka/pull/248/
 [gh-pull-0216]: https://github.com/moka-rs/moka/pull/216/

src/common/concurrent/deques.rs

@@ -194,3 +194,5 @@ impl<K> Deques<K> {
         }
     }
 }
+
+// TODO: Add tests and run Miri with them.

src/common/deque.rs

@@ -52,8 +52,8 @@ impl<T> DeqNode<T> {
         }
     }
 
-    pub(crate) fn next_node(&self) -> Option<&DeqNode<T>> {
-        self.next.as_ref().map(|node| unsafe { node.as_ref() })
+    pub(crate) fn next_node_ptr(this: NonNull<Self>) -> Option<NonNull<DeqNode<T>>> {
+        unsafe { this.as_ref() }.next
     }
 }
 
@@ -126,11 +126,13 @@ impl<T> Deque<T> {
     }
 
     pub(crate) fn peek_front(&self) -> Option<&DeqNode<T>> {
-        // This method takes care not to create mutable references to whole nodes,
-        // to maintain validity of aliasing pointers into `element`.
         self.head.as_ref().map(|node| unsafe { node.as_ref() })
     }
 
+    pub(crate) fn peek_front_ptr(&self) -> Option<NonNull<DeqNode<T>>> {
+        self.head.as_ref().cloned()
+    }
+
     /// Removes and returns the node at the front of the list.
     pub(crate) fn pop_front(&mut self) -> Option<Box<DeqNode<T>>> {
         // This method takes care not to create mutable references to whole nodes,
@@ -158,8 +160,6 @@ impl<T> Deque<T> {
     }
 
     pub(crate) fn peek_back(&self) -> Option<&DeqNode<T>> {
-        // This method takes care not to create mutable references to whole nodes,
-        // to maintain validity of aliasing pointers into `element`.
         self.tail.as_ref().map(|node| unsafe { node.as_ref() })
     }
 
@@ -221,6 +221,12 @@ impl<T> Deque<T> {
         }
     }
 
+    pub(crate) fn move_front_to_back(&mut self) {
+        if let Some(node) = self.head {
+            unsafe { self.move_to_back(node) };
+        }
+    }
+
     /// Unlinks the specified node from the current list.
     ///
     /// This method takes care not to create mutable references to `element`, to
@@ -683,35 +689,67 @@ mod tests {
         // -------------------------------------------------------
         // First iteration.
         // peek_front() -> node1
-        let node1a = deque.peek_front().unwrap();
-        assert_eq!(node1a.element, "a".to_string());
-        let node2a = node1a.next_node().unwrap();
-        assert_eq!(node2a.element, "b".to_string());
-        let node3a = node2a.next_node().unwrap();
-        assert_eq!(node3a.element, "c".to_string());
-        assert!(node3a.next_node().is_none());
+        let node1a = deque.peek_front_ptr().unwrap();
+        assert_eq!(unsafe { node1a.as_ref() }.element, "a".to_string());
+        let node2a = DeqNode::next_node_ptr(node1a).unwrap();
+        assert_eq!(unsafe { node2a.as_ref() }.element, "b".to_string());
+        let node3a = DeqNode::next_node_ptr(node2a).unwrap();
+        assert_eq!(unsafe { node3a.as_ref() }.element, "c".to_string());
+        assert!(DeqNode::next_node_ptr(node3a).is_none());
 
         // -------------------------------------------------------
         // Iterate after a move_to_back.
         // Move "b" to the back. So now "a" -> "c" -> "b".
         unsafe { deque.move_to_back(node2_ptr) };
-        let node1a = deque.peek_front().unwrap();
-        assert_eq!(node1a.element, "a".to_string());
-        let node3a = node1a.next_node().unwrap();
-        assert_eq!(node3a.element, "c".to_string());
-        let node2a = node3a.next_node().unwrap();
-        assert_eq!(node2a.element, "b".to_string());
-        assert!(node2a.next_node().is_none());
+        let node1a = deque.peek_front_ptr().unwrap();
+        assert_eq!(unsafe { node1a.as_ref() }.element, "a".to_string());
+        let node3a = DeqNode::next_node_ptr(node1a).unwrap();
+        assert_eq!(unsafe { node3a.as_ref() }.element, "c".to_string());
+        let node2a = DeqNode::next_node_ptr(node3a).unwrap();
+        assert_eq!(unsafe { node2a.as_ref() }.element, "b".to_string());
+        assert!(DeqNode::next_node_ptr(node2a).is_none());
 
         // -------------------------------------------------------
         // Iterate after an unlink.
         // Unlink the second node "c". Now "a" -> "c".
         unsafe { deque.unlink_and_drop(node3_ptr) };
-        let node1a = deque.peek_front().unwrap();
-        assert_eq!(node1a.element, "a".to_string());
-        let node2a = node1a.next_node().unwrap();
-        assert_eq!(node2a.element, "b".to_string());
-        assert!(node2a.next_node().is_none());
+        let node1a = deque.peek_front_ptr().unwrap();
+        assert_eq!(unsafe { node1a.as_ref() }.element, "a".to_string());
+        let node2a = DeqNode::next_node_ptr(node1a).unwrap();
+        assert_eq!(unsafe { node2a.as_ref() }.element, "b".to_string());
+        assert!(DeqNode::next_node_ptr(node2a).is_none());
+    }
+
+    #[test]
+    fn peek_and_move_to_back() {
+        let mut deque: Deque<String> = Deque::new(MainProbation);
+
+        let node1 = DeqNode::new("a".into());
+        deque.push_back(Box::new(node1));
+        let node2 = DeqNode::new("b".into());
+        let _ = deque.push_back(Box::new(node2));
+        let node3 = DeqNode::new("c".into());
+        let _ = deque.push_back(Box::new(node3));
+        // "a" -> "b" -> "c"
+
+        let node1a = deque.peek_front_ptr().unwrap();
+        assert_eq!(unsafe { node1a.as_ref() }.element, "a".to_string());
+        unsafe { deque.move_to_back(node1a) };
+        // "b" -> "c" -> "a"
+
+        let node2a = deque.peek_front_ptr().unwrap();
+        assert_eq!(unsafe { node2a.as_ref() }.element, "b".to_string());
+
+        let node3a = DeqNode::next_node_ptr(node2a).unwrap();
+        assert_eq!(unsafe { node3a.as_ref() }.element, "c".to_string());
+        unsafe { deque.move_to_back(node3a) };
+        // "b" -> "a" -> "c"
+
+        deque.move_front_to_back();
+        // "a" -> "c" -> "b"
+
+        let node1b = deque.peek_front().unwrap();
+        assert_eq!(node1b.element, "a".to_string());
     }
 
     #[test]

src/common/timer_wheel.rs

@@ -311,43 +311,31 @@ impl<K> TimerWheel<K> {
     /// Returns a pointer to the timer event (cache entry) at the front of the queue.
     /// Returns `None` if the front node is a sentinel.
     fn pop_timer_node(&mut self, level: usize, index: usize) -> Option<Box<DeqNode<TimerNode<K>>>> {
-        if let Some(node) = self.wheels[level][index].peek_front() {
+        let deque = &mut self.wheels[level][index];
+        if let Some(node) = deque.peek_front() {
             if node.element.is_sentinel() {
                 return None;
             }
         }
 
-        self.wheels[level][index].pop_front()
+        deque.pop_front()
     }
 
     /// Reset the positions of the nodes in the queue at the given level and index.
+    /// When done, the sentinel is at the back of the queue.
     fn reset_timer_node_positions(&mut self, level: usize, index: usize) {
-        if let Some(node) = self.wheels[level][index].peek_back() {
-            if node.element.is_sentinel() {
-                // The sentinel is at the back of the queue. We are already set.
-                return;
-            }
-        } else {
-            panic!(
-                "BUG: The queue is empty. level: {}, index: {}",
-                level, index
-            )
-        }
+        let deque = &mut self.wheels[level][index];
+        debug_assert!(
+            deque.len() > 0,
+            "BUG: The queue is empty. level: {}, index: {}",
+            level,
+            index
+        );
 
         // Rotate the nodes in the queue until we see the sentinel at the back of the
         // queue.
-        loop {
-            // Safe to unwrap because we already checked the queue is not empty.
-            let node = self.wheels[level][index].pop_front().unwrap();
-            let is_sentinel = node.element.is_sentinel();
-
-            // Move the front node to the back.
-            self.wheels[level][index].push_back(node);
-
-            // If the node we just moved was the sentinel, we are done.
-            if is_sentinel {
-                break;
-            }
+        while !deque.peek_back().unwrap().element.is_sentinel() {
+            deque.move_front_to_back();
         }
     }
 

src/sync_base/base_cache.rs

@@ -1682,10 +1682,6 @@ where
 
         // Move the skipped nodes to the back of the deque. We do not unlink (drop)
         // them because ValueEntries in the write op queue should be pointing them.
-        //
-        // TODO FIXME: This `move_to_back()` will be considered UB as violating the
-        // aliasing rule because these skipped nodes were acquired by `peek_front` or
-        // `next_node`. (They both return `&node` instead of `&mut node`).
         for node in skipped_nodes {
             unsafe { deqs.probation.move_to_back(node) };
         }
@@ -1723,26 +1719,26 @@ where
         let mut skipped_nodes = SmallVec::default();
 
         // Get first potential victim at the LRU position.
-        let mut next_victim = deqs.probation.peek_front();
+        let mut next_victim = deqs.probation.peek_front_ptr();
 
         // Aggregate potential victims.
         while victims.policy_weight < candidate.policy_weight {
             if candidate.freq < victims.freq {
                 break;
             }
             if let Some(victim) = next_victim.take() {
-                next_victim = victim.next_node();
-                let vic_elem = &victim.element;
+                next_victim = DeqNode::next_node_ptr(victim);
+                let vic_elem = &unsafe { victim.as_ref() }.element;
 
                 if let Some(vic_entry) = cache.get(vic_elem.hash(), |k| k == vic_elem.key()) {
                     victims.add_policy_weight(vic_entry.policy_weight());
                     victims.add_frequency(freq, vic_elem.hash());
-                    victim_nodes.push(NonNull::from(victim));
+                    victim_nodes.push(victim);
                     retries = 0;
                 } else {
                     // Could not get the victim from the cache (hash map). Skip this node
                     // as its ValueEntry might have been invalidated.
-                    skipped_nodes.push(NonNull::from(victim));
+                    skipped_nodes.push(victim);
 
                     retries += 1;
                     if retries > MAX_CONSECUTIVE_RETRIES {
@@ -2085,14 +2081,7 @@ where
             // invalidated ValueEntry (which should be still in the write op queue)
             // has a pointer to this node, move the node to the back of the deque
             // instead of popping (dropping) it.
-            //
-            // TODO FIXME: This `peek_front()` and `move_to_back()` combo will be
-            // considered UB as violating the aliasing rule. (`peek_front` returns
-            // `&node` instead of `&mut node`).
-            if let Some(node) = deq.peek_front() {
-                let node = NonNull::from(node);
-                unsafe { deq.move_to_back(node) };
-            }
+            deq.move_front_to_back();
             true
         }
     }
@@ -2157,14 +2146,7 @@ where
                 // invalidated ValueEntry (which should be still in the write op
                 // queue) has a pointer to this node, move the node to the back of
                 // the deque instead of popping (dropping) it.
-                //
-                // TODO FIXME: This `peek_front()` and `move_to_back()` combo will be
-                // considered UB as violating the aliasing rule (`peek_front` returns
-                // `&node` instead of `&mut node`).
-                if let Some(node) = deqs.write_order.peek_front() {
-                    let node = NonNull::from(node);
-                    unsafe { deqs.write_order.move_to_back(node) };
-                }
+                deqs.write_order.move_front_to_back();
             }
         }
     }