semaphore: convert to an intrusive list

Intrusive lists are faster since they require no allocations.

core/semaphore.cc

@@ -11,13 +11,12 @@ void semaphore::post(unsigned units)
         _val += units;
         auto i = _waiters.begin();
         while (_val > 0 && i != _waiters.end()) {
-            auto p = i++;
-            auto& wr = *p;
+            auto wr = i++;
             if (wr->units <= _val) {
                 _val -= wr->units;
                 wr->owner->wake();
                 wr->owner = nullptr;
-                _waiters.erase(p);
+                _waiters.erase(wr);
             }
         }
     });
@@ -34,7 +33,7 @@ void semaphore::wait(unsigned units)
         } else {
             wr.owner = sched::thread::current();
             wr.units = units;
-            _waiters.push_back(&wr);
+            _waiters.push_back(wr);
             wait = true;
         }
     });

include/osv/semaphore.hh

@@ -2,7 +2,7 @@
 #define SEMAPHORE_HH_
 
 #include <osv/mutex.h>
-#include <list>
+#include <boost/intrusive/list.hpp>
 #include <sched.hh>
 
 class semaphore {
@@ -14,11 +14,13 @@ public:
 private:
     unsigned _val;
     mutex _mtx;
-    struct wait_record {
+    struct wait_record : boost::intrusive::list_base_hook<> {
         sched::thread* owner;
         unsigned units;
     };
-    std::list<wait_record*> _waiters;
+    boost::intrusive::list<wait_record,
+                          boost::intrusive::base_hook<wait_record>,
+                          boost::intrusive::constant_time_size<false>> _waiters;
 };
 
 #endif /* SEMAPHORE_HH_ */