Current scope needs to be thread-safe for training

scope's API modifies its internal state. And scope's
API can be called from multiple threads during traing.
Hence, we need locks to protect the scope's internal
states.

We can optimize it in the future. But the current
solution is buggy.

test=develop

paddle/fluid/framework/scope.cc

@@ -20,13 +20,6 @@ limitations under the License. */
 #include "paddle/fluid/framework/threadpool.h"
 #include "paddle/fluid/string/printf.h"
 
-// The mutex is not needed by training and inference, only for distribution.
-#if PADDLE_WITH_DISTRIBUTE
-#define WITH_LOCK 1
-#else
-#define WITH_LOCK 0
-#endif
-
 DEFINE_bool(benchmark, false,
             "Doing memory benchmark. It will make deleting scope synchronized, "
             "and add some memory usage logs."
@@ -56,24 +49,18 @@ int64_t GetEagerDeletionThreshold() {
 Scope::~Scope() { DropKids(); }
 
 Scope& Scope::NewScope() const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   kids_.push_back(new Scope(this));
   return *kids_.back();
 }
 
 Variable* Scope::Var(const std::string& name) {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   return VarInternal(name);
 }
 
 Variable* Scope::Var(std::string* name) {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   auto new_name = string::Sprintf("%p.%d", this, vars_.size());
   if (name != nullptr) {
     *name = new_name;
@@ -82,39 +69,29 @@ Variable* Scope::Var(std::string* name) {
 }
 
 Variable* Scope::FindVar(const std::string& name) const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   return FindVarInternal(name);
 }
 
 const Scope* Scope::FindScope(const Variable* var) const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   return FindScopeInternal(var);
 }
 
 void Scope::DropKids() {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   for (Scope* s : kids_) delete s;
   kids_.clear();
 }
 
 bool Scope::HasKid(const Scope* scope) const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   auto it = std::find(this->kids_.begin(), this->kids_.end(), scope);
   return it != this->kids_.end();
 }
 
 std::vector<std::string> Scope::LocalVarNames() const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   std::vector<std::string> known_vars;
   known_vars.reserve(this->vars_.size());
   for (auto& p : vars_) {
@@ -124,9 +101,7 @@ std::vector<std::string> Scope::LocalVarNames() const {
 }
 
 void Scope::DeleteScope(Scope* scope) const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   auto it = std::find(this->kids_.begin(), this->kids_.end(), scope);
   PADDLE_ENFORCE(it != this->kids_.end(), "Cannot find %p as kid scope", scope);
   this->kids_.erase(it);
@@ -139,9 +114,7 @@ void Scope::DeleteScope(Scope* scope) const {
 }
 
 void Scope::EraseVars(const std::vector<std::string>& var_names) {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   std::set<std::string> var_set(var_names.begin(), var_names.end());
   for (auto it = vars_.begin(); it != vars_.end();) {
     if (var_set.find(it->first) != var_set.end()) {
@@ -154,16 +127,12 @@ void Scope::EraseVars(const std::vector<std::string>& var_names) {
 
 void Scope::Rename(const std::string& origin_name,
                    const std::string& new_name) const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   RenameInternal(origin_name, new_name);
 }
 
 std::string Scope::Rename(const std::string& origin_name) const {
-#if WITH_LOCK
   std::unique_lock<std::mutex> lock(mutex_);
-#endif
   auto new_name = string::Sprintf("%p.%d", this, vars_.size());
   RenameInternal(origin_name, new_name);
   return new_name;