squash! SelfContainedFutex: avoid live-lock in lock

sporksmith · sporksmith · commit 08fa4215fa9c · 2022-11-23T18:20:56.000-06:00
Use a single condvar per futex word.
diff --git a/src/lib/shadow-shim-helper-rs/src/scmutex.rs b/src/lib/shadow-shim-helper-rs/src/scmutex.rs
@@ -27,7 +27,7 @@ mod sync {
     use std::collections::HashMap;
     #[cfg(loom)]
     loom::lazy_static! {
-        pub static ref FUTEXES: Mutex<HashMap<usize, Vec<Arc<Condvar>>>> = Mutex::new(HashMap::new());
+        pub static ref FUTEXES: Mutex<HashMap<usize, Arc<Condvar>>> = Mutex::new(HashMap::new());
     }
 
     #[cfg(not(loom))]
@@ -68,19 +68,18 @@ mod sync {
         if futex_word_val != val {
             return Err(nix::errno::Errno::EAGAIN);
         }
-        let waiters = hashmap
+        let condvar = hashmap
             .entry(futex_word as *const _ as usize)
-            .or_insert(Vec::new());
-        let condvar = Arc::new(Condvar::new());
-        waiters.push(condvar.clone());
+            .or_insert(Arc::new(Condvar::new()))
+            .clone();
         // We could get a spurious wakeup here, but that's ok.
         // Futexes are subject to spurious wakeups too.
         condvar.wait(hashmap).unwrap();
         Ok(0)
     }
 
     #[cfg(not(loom))]
-    pub(super) unsafe fn futex_wake(futex_word: &super::AtomicFutexWord) -> nix::Result<i64> {
+    pub(super) unsafe fn futex_wake(futex_word: &super::AtomicFutexWord) -> nix::Result<()> {
         // The kernel just checks for equality of the value at this pointer.
         // Just validate that it's the right size and alignment.
         assert_eq!(
@@ -103,19 +102,17 @@ mod sync {
                 std::ptr::null_mut() as *mut u32,
                 032,
             )
-        })
+        })?;
+        Ok(())
     }
     #[cfg(loom)]
-    pub(super) unsafe fn futex_wake(futex_word: &super::AtomicFutexWord) -> nix::Result<i64> {
-        let mut hashmap = FUTEXES.lock().unwrap();
-        let Some(waiters) = hashmap.get_mut(&(futex_word as *const _ as usize)) else {
-            return Ok(0);
-        };
-        let Some(condvar) = waiters.pop() else {
-            return Ok(0);
+    pub(super) unsafe fn futex_wake(futex_word: &super::AtomicFutexWord) -> nix::Result<()> {
+        let hashmap = FUTEXES.lock().unwrap();
+        let Some(condvar) = hashmap.get(&(futex_word as *const _ as usize)) else {
+            return Ok(());
         };
-        condvar.notify_all();
-        Ok(1)
+        condvar.notify_one();
+        Ok(())
     }
 
     #[cfg(not(loom))]
