fix thread-safety bug

test=develop

paddle/fluid/platform/device_context.cc

@@ -120,14 +120,25 @@ class EigenCudaStreamDevice : public Eigen::StreamInterface {
   }
 
   void* allocate(size_t num_bytes) const override {
+    if (UNLIKELY(num_bytes == 0)) {
+      return nullptr;
+    }
     auto buf = paddle::memory::Alloc(place_, num_bytes,
                                      memory::Allocator::kScratchpad);
     void* retv = buf->ptr();
-    allocations_[buf->ptr()] = std::move(buf);
+    {
+      std::lock_guard<std::mutex> lock(mtx_);
+      allocations_.emplace(retv, std::move(buf));
+    }
     return retv;
   }
 
-  void deallocate(void* buffer) const override { allocations_.erase(buffer); }
+  void deallocate(void* buffer) const override {
+    if (LIKELY(buffer)) {
+      std::lock_guard<std::mutex> lock(mtx_);
+      allocations_.erase(buffer);
+    }
+  }
 
   void* scratchpad() const override {
     if (scratch_ == NULL) {
@@ -153,6 +164,7 @@ class EigenCudaStreamDevice : public Eigen::StreamInterface {
   const cudaDeviceProp* device_prop_;  // not owned;
   mutable void* scratch_;
   mutable unsigned int* semaphore_;
+  mutable std::mutex mtx_;  // to protect allocations_
   mutable std::unordered_map<void*, memory::AllocationPtr> allocations_;
 };