Removed inappropriate use of SerialTaskQueue from ClusterShapeLazyGetter

A SerialTaskQueue was added to ClusterShapeLazyGetter in an attempt
to make it thread safe. Unfortunately, the implementation was not safe
since the internals of SiPixelClusterShapeCache could be changed
while another thread was reading the data outside of the queue.
In addition, performance measurements done using VTune showed
the fequent calls to SerialTaskQueue were causing a heavy CPU load
when run with 8 threads.
The thread safety of SiPixelClusterShapeCache will have to be handled
in a different way in the future once we start using more than one
thread per Event.

RecoPixelVertexing/PixelLowPtUtilities/plugins/SiPixelClusterShapeCacheProducer.cc

@@ -3,7 +3,6 @@
 #include "FWCore/Framework/interface/Event.h"
 #include "FWCore/Framework/interface/EventSetup.h"
 #include "FWCore/Utilities/interface/EDGetToken.h"
-#include "FWCore/Concurrency/interface/SerialTaskQueue.h"
 
 #include "FWCore/ParameterSet/interface/ParameterSet.h"
 #include "FWCore/Utilities/interface/InputTag.h"
@@ -31,18 +30,15 @@ namespace {
     ~ClusterShapeLazyGetter() {}
 
     void fill(const SiPixelClusterShapeCache::ClusterRef& cluster, const PixelGeomDetUnit *pixDet, const SiPixelClusterShapeCache& constCache) const override {
-      taskQueue_.pushAndWait([this, &cluster, pixDet, &constCache]{
-          if(constCache.isFilled(cluster))
-            return;
-          SiPixelClusterShapeCache& cache = const_cast<SiPixelClusterShapeCache&>(constCache);
-          this->data_.size.clear();
-          this->clusterShape_.determineShape(*pixDet, *cluster, this->data_);
-          cache.insert(cluster, this->data_);
-        });
+      if(constCache.isFilled(cluster))
+        return;
+      SiPixelClusterShapeCache& cache = const_cast<SiPixelClusterShapeCache&>(constCache);
+      this->data_.size.clear();
+      this->clusterShape_.determineShape(*pixDet, *cluster, this->data_);
+      cache.insert(cluster, this->data_);
     }
 
   private:
-    mutable edm::SerialTaskQueue taskQueue_; // not sure if this is the best synchronization mechanism
     mutable ClusterData data_; // reused
     mutable ClusterShape clusterShape_;
   };