From ea2914d835c7590196b491a32bed324d4c74b95f Mon Sep 17 00:00:00 2001
From: Jacob Greenfield <jacob@jacobgreenfield.me>
Date: Wed, 16 Oct 2024 16:40:09 -0400
Subject: [PATCH 1/4] Return `Some` for only one thread when calling `remove()`

---
 crossbeam-skiplist/src/base.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/crossbeam-skiplist/src/base.rs b/crossbeam-skiplist/src/base.rs
index f3c90b1f9..af68123dd 100644
--- a/crossbeam-skiplist/src/base.rs
+++ b/crossbeam-skiplist/src/base.rs
@@ -1157,8 +1157,12 @@ where
                             break;
                         }
                     }
+                    return Some(entry);
+                } else {
+                    // The node has already been marked.
+                    n.decrement(guard);
+                    return None;
                 }
-                return Some(entry);
             }
         }
     }

From 769c520f16fc62a25fe6ea0e8c2abde8cd0c4f65 Mon Sep 17 00:00:00 2001
From: Jacob Greenfield <jacob@jacobgreenfield.me>
Date: Thu, 19 Dec 2024 19:40:39 -0500
Subject: [PATCH 2/4] Add test for remove() race condition

---
 crossbeam-skiplist/tests/base.rs | 50 ++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/crossbeam-skiplist/tests/base.rs b/crossbeam-skiplist/tests/base.rs
index 83d2f221e..14e41a805 100644
--- a/crossbeam-skiplist/tests/base.rs
+++ b/crossbeam-skiplist/tests/base.rs
@@ -900,3 +900,53 @@ fn drops() {
     assert_eq!(KEYS.load(Ordering::SeqCst), 8);
     assert_eq!(VALUES.load(Ordering::SeqCst), 7);
 }
+
+// https://github.com/crossbeam-rs/crossbeam/pull/1143
+#[cfg(target_has_atomic = "32")]
+#[test]
+fn remove_race() {
+    use std::sync::atomic::{AtomicU32, Ordering};
+
+    let nthreads = 16;
+    let key_range = 100_000u32;
+
+    let guard = &epoch::pin();
+    let s = SkipList::new(epoch::default_collector().clone());
+
+    for x in 0..key_range {
+        s.insert(x, (), guard);
+    }
+
+    let barrier1 = AtomicU32::new(nthreads);
+    let barrier2 = AtomicU32::new(nthreads);
+    let mut total_removed = AtomicU32::new(0);
+
+    std::thread::scope(|scope| {
+        for _ in 0..nthreads {
+            scope.spawn(|| {
+                let guard = &epoch::pin();
+                let mut removed_entries = Vec::with_capacity(key_range as usize);
+
+                barrier1.fetch_sub(1, Ordering::Relaxed);
+                while barrier1.load(Ordering::Acquire) != 0 {}
+
+                for x in 0..key_range {
+                    if let Some(entry) = s.remove(&x, guard) {
+                        removed_entries.push(entry);
+                    }
+                }
+
+                barrier2.fetch_sub(1, Ordering::Relaxed);
+                while barrier2.load(Ordering::Acquire) != 0 {}
+
+                total_removed.fetch_add(removed_entries.len() as u32, Ordering::Relaxed);
+
+                for entry in removed_entries.drain(..) {
+                    entry.release(guard);
+                }
+            });
+        }
+    });
+
+    assert_eq!(*total_removed.get_mut(), key_range);
+}

From ea0fbbd66fd083679e7ca7bb0285715af1e2084a Mon Sep 17 00:00:00 2001
From: Jacob Greenfield <jacob@jacobgreenfield.me>
Date: Thu, 19 Dec 2024 20:27:53 -0500
Subject: [PATCH 3/4] Smaller key range for miri

---
 crossbeam-skiplist/tests/base.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/crossbeam-skiplist/tests/base.rs b/crossbeam-skiplist/tests/base.rs
index 1d4043f28..8abcc29e9 100644
--- a/crossbeam-skiplist/tests/base.rs
+++ b/crossbeam-skiplist/tests/base.rs
@@ -978,6 +978,9 @@ fn remove_race() {
     use std::sync::atomic::{AtomicU32, Ordering};
 
     let nthreads = 16;
+    #[cfg(miri)]
+    let key_range = 100u32;
+    #[cfg(not(miri))]
     let key_range = 100_000u32;
 
     let guard = &epoch::pin();

From f9369988bc1fe1951a1162e180e84c94c66b1cb9 Mon Sep 17 00:00:00 2001
From: Jacob Greenfield <jacob@jacobgreenfield.me>
Date: Thu, 19 Dec 2024 20:35:57 -0500
Subject: [PATCH 4/4] Fix memory leak

---
 crossbeam-skiplist/tests/base.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crossbeam-skiplist/tests/base.rs b/crossbeam-skiplist/tests/base.rs
index 8abcc29e9..d12b7fde7 100644
--- a/crossbeam-skiplist/tests/base.rs
+++ b/crossbeam-skiplist/tests/base.rs
@@ -987,7 +987,7 @@ fn remove_race() {
     let s = SkipList::new(epoch::default_collector().clone());
 
     for x in 0..key_range {
-        s.insert(x, (), guard);
+        s.insert(x, (), guard).release(guard);
     }
 
     let barrier1 = AtomicU32::new(nthreads);