Add "scheduler" / "locking_spin_count" (int) config parameter.

asio/include/asio/detail/conditionally_enabled_mutex.hpp

@@ -70,6 +70,14 @@ class conditionally_enabled_mutex
     {
       if (mutex_.enabled_ && !locked_)
       {
+        for (int n = mutex_.spin_count_; n != 0; n -= (n > 0) ? 1 : 0)
+        {
+          if (mutex_.mutex_.try_lock())
+          {
+            locked_ = true;
+            return;
+          }
+        }
         mutex_.mutex_.lock();
         locked_ = true;
       }
@@ -104,8 +112,9 @@ class conditionally_enabled_mutex
   };
 
   // Constructor.
-  explicit conditionally_enabled_mutex(bool enabled)
-    : enabled_(enabled)
+  explicit conditionally_enabled_mutex(bool enabled, int spin_count = 0)
+    : spin_count_(spin_count),
+      enabled_(enabled)
   {
   }
 
@@ -124,7 +133,12 @@ class conditionally_enabled_mutex
   void lock()
   {
     if (enabled_)
+    {
+      for (int n = spin_count_; n != 0; n -= (n > 0) ? 1 : 0)
+        if (mutex_.try_lock())
+          return;
       mutex_.lock();
+    }
   }
 
   // Unlock the mutex.
@@ -138,6 +152,7 @@ class conditionally_enabled_mutex
   friend class scoped_lock;
   friend class conditionally_enabled_event;
   asio::detail::mutex mutex_;
+  const int spin_count_;
   const bool enabled_;
 };
 

asio/include/asio/detail/impl/scheduler.ipp

@@ -112,7 +112,8 @@ scheduler::scheduler(asio::execution_context& ctx,
     bool own_thread, get_task_func_type get_task)
   : asio::detail::execution_context_service_base<scheduler>(ctx),
     one_thread_(config(ctx).get("scheduler", "concurrency_hint", 0) == 1),
-    mutex_(config(ctx).get("scheduler", "locking", true)),
+    mutex_(config(ctx).get("scheduler", "locking", true),
+        config(ctx).get("scheduler", "locking_spin_count", 0)),
     task_(0),
     get_task_(get_task),
     task_interrupted_(true),

asio/include/asio/detail/null_static_mutex.hpp

@@ -35,6 +35,12 @@ struct null_static_mutex
   {
   }
 
+  // Try to lock the mutex without blocking.
+  bool try_lock()
+  {
+    return true;
+  }
+
   // Lock the mutex.
   void lock()
   {

asio/include/asio/detail/posix_mutex.hpp

@@ -45,6 +45,12 @@ class posix_mutex
     ::pthread_mutex_destroy(&mutex_); // Ignore EBUSY.
   }
 
+  // Try to lock the mutex.
+  bool try_lock()
+  {
+    return ::pthread_mutex_trylock(&mutex_) == 0; // Ignore EINVAL.
+  }
+
   // Lock the mutex.
   void lock()
   {

asio/include/asio/detail/std_mutex.hpp

@@ -43,6 +43,12 @@ class std_mutex
   {
   }
 
+  // Try to lock the mutex.
+  bool try_lock()
+  {
+    return mutex_.try_lock();
+  }
+
   // Lock the mutex.
   void lock()
   {

asio/include/asio/detail/win_mutex.hpp

@@ -43,6 +43,12 @@ class win_mutex
     ::DeleteCriticalSection(&crit_section_);
   }
 
+  // Try to lock the mutex.
+  bool try_lock()
+  {
+    return ::TryEnterCriticalSection(&crit_section_) != 0;
+  }
+
   // Lock the mutex.
   void lock()
   {