1 files changed, 17 insertions, 25 deletions

diff --git a/include/salticidae/queue.h b/include/salticidae/queue.h
index 598303c..a72a28f 100644
--- a/include/salticidae/queue.h
+++ b/include/salticidae/queue.h
@@ -13,9 +13,8 @@ class FreeList {
     public:
     struct Node {
         std::atomic<Node *> next;
-        std::atomic<bool> addback;
         std::atomic<size_t> refcnt;
-        Node(): next(nullptr), addback(false), refcnt(1) {}
+        Node(): next(nullptr), refcnt(1) {}
     };
 
     private:
@@ -28,16 +27,14 @@ class FreeList {
 
     void release_ref(Node *u) {
         if (u->refcnt.fetch_sub(1, std::memory_order_relaxed) != 1) return;
-        u->addback.store(false, std::memory_order_relaxed);
-        //if (!u->addback.exchange(false, std::memory_order_relaxed)) return;
         for (;;)
         {
-            auto t = top.load(std::memory_order_acquire);
+            auto t = top.load(std::memory_order_relaxed);
             // repair the next pointer before CAS, otherwise u->next == nullptr
             // could lead to skipping elements
-            u->next.store(t, std::memory_order_consume);
+            u->next.store(t, std::memory_order_relaxed);
             // the replacement is ok even if ABA happens
-            if (top.compare_exchange_weak(t, u, std::memory_order_consume))
+            if (top.compare_exchange_weak(t, u, std::memory_order_release))
             {
                 u->refcnt.store(1, std::memory_order_relaxed);
                 break;
@@ -46,9 +43,6 @@ class FreeList {
     }
 
     bool push(Node *u) {
-        assert(u->addback.load() == false);
-        // attempt to push it
-        u->addback.store(true, std::memory_order_release);
         release_ref(u);
         return true;
     }
@@ -60,18 +54,18 @@ class FreeList {
             auto u = top.load(std::memory_order_acquire);
             /* the list is now empty */
             if (u == nullptr) return false;
-            auto t = u->refcnt.load(std::memory_order_acquire);
+            auto t = u->refcnt.load(std::memory_order_relaxed);
             /* let's wait for another round if u is a ghost (already popped) */
             if (!t) continue;
             /* otherwise t > 0, so with CAS, the invariant that zero refcnt can
              * never be increased is guaranteed */
-            if (u->refcnt.compare_exchange_weak(t, t + 1, std::memory_order_consume))
+            if (u->refcnt.compare_exchange_weak(t, t + 1, std::memory_order_relaxed))
             {
                 /* here, nobody is able to change v->next (because v->next is
                  * only changed when pushed) even when ABA happens */
                 auto v = u;
-                auto nv = u->next.load(std::memory_order_acquire);
-                if (top.compare_exchange_weak(v, nv, std::memory_order_consume))
+                auto nv = u->next.load(std::memory_order_relaxed);
+                if (top.compare_exchange_weak(v, nv, std::memory_order_relaxed))
                 {
                     /* manage to pop the head */
                     r = u;
@@ -104,7 +98,6 @@ class MPMCQueue {
     template<typename U>
     void _enqueue(Block *nblk, U &&e) {
         new (&(nblk->elem)) T(std::forward<U>(e));
-        std::atomic_thread_fence(std::memory_order_release);
         nblk->next.store(nullptr, std::memory_order_relaxed);
         auto prev = tail.exchange(nblk, std::memory_order_acq_rel);
         prev->next.store(nblk, std::memory_order_relaxed);
@@ -151,7 +144,7 @@ class MPMCQueue {
             auto h = head.load(std::memory_order_relaxed);
             auto t = h->refcnt.load(std::memory_order_relaxed);
             if (!t) continue;
-            if (h->refcnt.compare_exchange_weak(t, t + 1, std::memory_order_consume))
+            if (h->refcnt.compare_exchange_weak(t, t + 1, std::memory_order_relaxed))
             {
                 auto nh = h->next.load(std::memory_order_relaxed);
                 if (nh == nullptr)
@@ -159,10 +152,9 @@ class MPMCQueue {
                     blks.release_ref(h);
                     return false;
                 }
-                std::atomic_thread_fence(std::memory_order_acquire);
                 e = std::move(nh->elem);
                 auto hh = h;
-                if (head.compare_exchange_weak(hh, nh, std::memory_order_consume))
+                if (head.compare_exchange_weak(hh, nh, std::memory_order_relaxed))
                 {
                     blks.release_ref(h);
                     blks.push(h);
@@ -177,13 +169,13 @@ class MPMCQueue {
 template<typename T>
 struct MPSCQueue: public MPMCQueue<T> {
     using MPMCQueue<T>::MPMCQueue;
+    /* the same thread is calling the following functions */
+
     bool try_dequeue(T &e) {
         auto h = this->head.load(std::memory_order_relaxed);
         auto nh = h->next.load(std::memory_order_relaxed);
-        std::atomic_thread_fence(std::memory_order_acquire);
         if (nh == nullptr) return false;
         e = std::move(nh->elem);
-        std::atomic_thread_fence(std::memory_order_release);
         this->head.store(nh, std::memory_order_relaxed);
         this->blks.push(h);
         return true;
@@ -192,13 +184,13 @@ struct MPSCQueue: public MPMCQueue<T> {
     template<typename U>
     bool rewind(U &&e) {
         FreeList::Node * _nblk;
-        if (!this->blks.pop(_nblk)) return false;
+        if (!this->blks.pop(_nblk)) _nblk = new typename MPMCQueue<T>::Block();
+
         auto nblk = static_cast<typename MPMCQueue<T>::Block *>(_nblk);
-        auto h = this->head.load(std::memory_order_acquire);
-        nblk->next.store(h, std::memory_order_release);
+        auto h = this->head.load(std::memory_order_relaxed);
         new (&(h->elem)) T(std::forward<U>(e));
-        std::atomic_thread_fence(std::memory_order_release);
-        this->head.store(nblk, std::memory_order_release);
+        nblk->next.store(h, std::memory_order_relaxed);
+        this->head.store(nblk, std::memory_order_relaxed);
         return true;
     }
 };