Fix a data race in microbenchmark stats gathering

test/core/util/passthru_endpoint.c

@@ -82,7 +82,7 @@ static void me_write(grpc_exec_ctx *exec_ctx, grpc_endpoint *ep,
   half *m = other_half((half *)ep);
   gpr_mu_lock(&m->parent->mu);
   grpc_error *error = GRPC_ERROR_NONE;
-  m->parent->stats->num_writes++;
+  gpr_atm_no_barrier_fetch_add(&m->parent->stats->num_writes, (gpr_atm)1);
   if (m->parent->shutdown) {
     error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Endpoint already shutdown");
   } else if (m->on_read != NULL) {

test/core/util/passthru_endpoint.h

@@ -19,9 +19,11 @@
 #ifndef MOCK_ENDPOINT_H
 #define MOCK_ENDPOINT_H
 
+#include <grpc/support/atm.h>
+
 #include "src/core/lib/iomgr/endpoint.h"
 
-typedef struct { int num_writes; } grpc_passthru_endpoint_stats;
+typedef struct { gpr_atm num_writes; } grpc_passthru_endpoint_stats;
 
 void grpc_passthru_endpoint_create(grpc_endpoint **client,
                                    grpc_endpoint **server,

test/cpp/microbenchmarks/fullstack_fixtures.h

@@ -25,6 +25,7 @@
 #include <grpc++/security/server_credentials.h>
 #include <grpc++/server.h>
 #include <grpc++/server_builder.h>
+#include <grpc/support/atm.h>
 #include <grpc/support/log.h>
 
 extern "C" {
@@ -259,7 +260,8 @@ class InProcessCHTTP2 : public EndpointPairFixture {
   void AddToLabel(std::ostream& out, benchmark::State& state) {
     EndpointPairFixture::AddToLabel(out, state);
     out << " writes/iter:"
-        << (double)stats_.num_writes / (double)state.iterations();
+        << static_cast<double>(gpr_atm_no_barrier_load(&stats_.num_writes)) /
+               static_cast<double>(state.iterations());
   }
 
  private: