Fix race in phase lock contention counting

3 files changed, 12 insertions, 9 deletions

diff --git a/build2/b.cxx b/build2/b.cxx
index e615ef5..b14da09 100644
--- a/build2/b.cxx
+++ b/build2/b.cxx
@@ -423,7 +423,8 @@ main (int argc, char* argv[])
     {
       if (pctx != nullptr)
       {
-        phase_switch_contention += pctx->phase_mutex.contention;
+        phase_switch_contention += (pctx->phase_mutex.contention +
+                                    pctx->phase_mutex.contention_load);
         pctx = nullptr; // Free first to reuse memory.
       }
 
@@ -1402,7 +1403,8 @@ main (int argc, char* argv[])
     }
 #endif
 
-    phase_switch_contention += pctx->phase_mutex.contention;
+    phase_switch_contention += (pctx->phase_mutex.contention +
+                                pctx->phase_mutex.contention_load);
   }
   catch (const failed&)
   {
diff --git a/libbuild2/context.cxx b/libbuild2/context.cxx
index d460fa7..8052716 100644
--- a/libbuild2/context.cxx
+++ b/libbuild2/context.cxx
@@ -765,7 +765,7 @@ namespace build2
       }
       else if (ctx_.phase != n)
       {
-        ++contention;
+        ++contention; // Protected by m_.
 
         ctx_.sched.deactivate (false /* external */);
         for (; ctx_.phase != n; v->wait (l)) ;
@@ -783,11 +783,11 @@ namespace build2
     {
       if (!lm_.try_lock ())
       {
-        ++contention;
-
         ctx_.sched.deactivate (false /* external */);
         lm_.lock ();
         ctx_.sched.activate (false /* external */);
+
+        ++contention_load; // Protected by lm_.
       }
       r = !fail_; // Re-query.
     }
@@ -905,7 +905,7 @@ namespace build2
       }
       else // phase != n
       {
-        ++contention;
+        ++contention; // Protected by m_.
 
         ctx_.sched.deactivate (false /* external */);
         for (; ctx_.phase != n; v->wait (l)) ;
@@ -919,11 +919,11 @@ namespace build2
     {
       if (!lm_.try_lock ())
       {
-        ++contention;
-
         ctx_.sched.deactivate (false /* external */);
         lm_.lock ();
         ctx_.sched.activate (false /* external */);
+
+        ++contention_load; // Protected by lm_.
       }
       r = !fail_; // Re-query.
     }
diff --git a/libbuild2/context.hxx b/libbuild2/context.hxx
index 0595413..fbdf9b8 100644
--- a/libbuild2/context.hxx
+++ b/libbuild2/context.hxx
@@ -46,7 +46,8 @@ namespace build2
     // Statistics.
     //
   public:
-    size_t contention = 0; // # of contentious phase (re)locks.
+    size_t contention      = 0; // # of contentious phase (re)locks.
+    size_t contention_load = 0; // # of contentious load phase locks.
 
   private:
     friend class context;