Simplify Mark processing code

test/cpp/qps/client.h

@@ -123,15 +123,13 @@ class Client {
     if (reset) {
       Histogram* to_merge = new Histogram[threads_.size()];
       for (size_t i = 0; i < threads_.size(); i++) {
-        threads_[i]->BeginSwap(&to_merge[i]);
-      }
-      std::unique_ptr<UsageTimer> timer(new UsageTimer);
-      timer_.swap(timer);
-      for (size_t i = 0; i < threads_.size(); i++) {
-        threads_[i]->EndSwap();
+        threads_[i]->Swap(&to_merge[i]);
         latencies.Merge(to_merge[i]);
       }
       delete[] to_merge;
+
+      std::unique_ptr<UsageTimer> timer(new UsageTimer);
+      timer_.swap(timer);
       timer_result = timer->Mark();
     } else {
       // merge snapshots of each thread histogram
@@ -227,7 +225,6 @@ class Client {
    public:
     Thread(Client* client, size_t idx)
         : done_(false),
-          new_stats_(nullptr),
           client_(client),
           idx_(idx),
           impl_(&Thread::ThreadFunc, this) {}
@@ -240,16 +237,9 @@ class Client {
       impl_.join();
     }
 
-    void BeginSwap(Histogram* n) {
+    void Swap(Histogram* n) {
       std::lock_guard<std::mutex> g(mu_);
-      new_stats_ = n;
-    }
-
-    void EndSwap() {
-      std::unique_lock<std::mutex> g(mu_);
-      while (new_stats_ != nullptr) {
-        cv_.wait(g);
-      };
+      n->Swap(&histogram_);
     }
 
     void MergeStatsInto(Histogram* hist) {
@@ -263,10 +253,11 @@ class Client {
 
     void ThreadFunc() {
       for (;;) {
+	// lock since the thread should only be doing one thing at a time
+        std::lock_guard<std::mutex> g(mu_);
         // run the loop body
         const bool thread_still_ok = client_->ThreadFunc(&histogram_, idx_);
-        // lock, see if we're done
-        std::lock_guard<std::mutex> g(mu_);
+        // see if we're done
         if (!thread_still_ok) {
           gpr_log(GPR_ERROR, "Finishing client thread due to RPC error");
           done_ = true;
@@ -274,19 +265,11 @@ class Client {
         if (done_) {
           return;
         }
-        // check if we're resetting stats, swap out the histogram if so
-        if (new_stats_) {
-          new_stats_->Swap(&histogram_);
-          new_stats_ = nullptr;
-          cv_.notify_one();
-        }
       }
     }
 
     std::mutex mu_;
-    std::condition_variable cv_;
     bool done_;
-    Histogram* new_stats_;
     Histogram histogram_;
     Client* client_;
     const size_t idx_;