Fix memory ordering of WorkerAvailability (#340)

Author: fakeshadow

actix-server/src/accept.rs

@@ -128,7 +128,7 @@ impl Availability {
     /// This would result in a re-check on all workers' availability.
     fn set_available_all(&mut self, handles: &[WorkerHandleAccept]) {
         handles.iter().for_each(|handle| {
-            self.set_available(handle.idx, true);
+            self.set_available(handle.idx(), true);
         })
     }
 }
@@ -248,7 +248,7 @@ impl Accept {
                                 drop(guard);
                                 // maybe we want to recover from a backpressure.
                                 self.maybe_backpressure(&mut sockets, false);
-                                self.avail.set_available(handle.idx, true);
+                                self.avail.set_available(handle.idx(), true);
                                 self.handles.push(handle);
                             }
                             // got timer interest and it's time to try register socket(s) again
@@ -400,7 +400,7 @@ impl Accept {
         } else {
             while self.avail.available() {
                 let next = self.next();
-                let idx = next.idx;
+                let idx = next.idx();
                 if next.available() {
                     self.avail.set_available(idx, true);
                     match self.send_connection(sockets, conn) {
@@ -503,7 +503,7 @@ impl Accept {
     /// Remove next worker handle that fail to accept connection.
     fn remove_next(&mut self) {
         let handle = self.handles.swap_remove(self.next);
-        let idx = handle.idx;
+        let idx = handle.idx();
         // A message is sent to `ServerBuilder` future to notify it a new worker
         // should be made.
         self.srv.worker_faulted(idx);

actix-server/src/worker.rs

@@ -47,11 +47,7 @@ fn handle_pair(
     tx2: UnboundedSender<Stop>,
     avail: WorkerAvailability,
 ) -> (WorkerHandleAccept, WorkerHandleServer) {
-    let accept = WorkerHandleAccept {
-        idx,
-        tx: tx1,
-        avail,
-    };
+    let accept = WorkerHandleAccept { tx: tx1, avail };
 
     let server = WorkerHandleServer { idx, tx: tx2 };
 
@@ -63,16 +59,22 @@ fn handle_pair(
 ///
 /// Held by [Accept](crate::accept::Accept).
 pub(crate) struct WorkerHandleAccept {
-    pub idx: usize,
     tx: UnboundedSender<Conn>,
     avail: WorkerAvailability,
 }
 
 impl WorkerHandleAccept {
+    #[inline(always)]
+    pub(crate) fn idx(&self) -> usize {
+        self.avail.idx
+    }
+
+    #[inline(always)]
     pub(crate) fn send(&self, msg: Conn) -> Result<(), Conn> {
         self.tx.send(msg).map_err(|msg| msg.0)
     }
 
+    #[inline(always)]
     pub(crate) fn available(&self) -> bool {
         self.avail.available()
     }
@@ -110,13 +112,18 @@ impl WorkerAvailability {
         }
     }
 
+    #[inline(always)]
     pub fn available(&self) -> bool {
         self.available.load(Ordering::Acquire)
     }
 
     pub fn set(&self, val: bool) {
-        let old = self.available.swap(val, Ordering::Release);
-        // notify the accept on switched to available.
+        // Ordering:
+        //
+        // There could be multiple set calls happen in one <ServerWorker as Future>::poll.
+        // Order is important between them.
+        let old = self.available.swap(val, Ordering::AcqRel);
+        // Notify the accept on switched to available.
         if !old && val {
             self.waker.wake(WakerInterest::WorkerAvailable(self.idx));
         }
@@ -374,6 +381,10 @@ impl Default for WorkerState {
 
 impl Drop for ServerWorker {
     fn drop(&mut self) {
+        // Set availability to true so if accept try to send connection to this worker
+        // it would find worker is gone and remove it.
+        // This is helpful when worker is dropped unexpected.
+        self.availability.set(true);
         // Stop the Arbiter ServerWorker runs on on drop.
         Arbiter::current().stop();
     }