contenting mutex (alibaba#992)

Author: lihuiba

thread/thread.cpp

@@ -1725,7 +1725,7 @@ R"(
         {
             assert(h->waitq == (thread_list*)this);
             prelocked_thread_interrupt(h, error_number);
-            assert(h->waitq == nullptr);
+            // assert(h->waitq == nullptr);
             assert(this->q.th != h);
         }
         return h;
@@ -1750,6 +1750,7 @@ R"(
             if (unlikely(ret)) { errno = ret; return -1; }
             if (try_lock() == 0) return 0;
         }
+    again:
         splock.lock();
         if (try_lock() == 0) {
             splock.unlock();
@@ -1764,6 +1765,10 @@ R"(
 
         int ret = thread_usleep_defer(timeout,
             (thread_list*)&q, &spinlock_unlock, &splock);
+        if (likely(ret < 0 && errno == -1)) {
+            auto o = owner.load(std::memory_order_acquire);
+            if (unlikely(o != CURRENT)) { assert(_contending); goto again; }
+        }
         return waitq_translate_errno(ret);
     }
     int mutex::try_lock()
@@ -1777,7 +1782,7 @@ R"(
     {
         SCOPED_LOCK(m->splock);
         ScopedLockHead h(m);
-        m->owner.store(h);
+        m->owner.store(unlikely(m->_contending) ? nullptr : (thread*)h);
         if (h)
             prelocked_thread_interrupt(h, -1);
     }

thread/thread.h

@@ -302,7 +302,8 @@ namespace photon
     class mutex : protected waitq
     {
     public:
-        mutex(uint16_t max_retries = 100) : retries(max_retries) { }
+        mutex(uint16_t max_retries = 100, bool contending = false)
+            : retries(max_retries), _contending(contending) { }
         int lock(Timeout timeout = {});
         int try_lock();
         void unlock();
@@ -315,6 +316,7 @@ namespace photon
         std::atomic<thread*> owner{nullptr};
         uint16_t retries;
         spinlock splock;
+        bool _contending;
     };
 
     class seq_mutex : protected mutex {