diff --git a/BUILD b/BUILD
index a059ff3e611..5aee6cc12d1 100644
--- a/BUILD
+++ b/BUILD
@@ -2177,7 +2177,10 @@ grpc_cc_library(
     hdrs = [
         "src/core/lib/iomgr/error.h",
     ],
-    external_deps = ["absl/status"],
+    external_deps = [
+        "absl/status",
+        "absl/strings:str_format",
+    ],
     deps = [
         "gpr",
         "gpr_spinlock",
diff --git a/src/core/lib/iomgr/error.cc b/src/core/lib/iomgr/error.cc
index cd0b6cacb56..9d6e59230cd 100644
--- a/src/core/lib/iomgr/error.cc
+++ b/src/core/lib/iomgr/error.cc
@@ -22,6 +22,8 @@
 #include <inttypes.h>
 #include <string.h>
 
+#include "absl/strings/str_format.h"
+
 #include <grpc/impl/codegen/status.h>
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
@@ -65,13 +67,32 @@ std::string grpc_error_std_string(absl::Status error) {
   return grpc_core::StatusToString(error);
 }
 
+namespace {
+#ifdef GPR_WINDOWS
+std::string StrError(int err) { return strerror(err); }
+#else
+std::string StrError(int err) {
+  struct Finish {
+    static std::string Run(char* buf, int err, int r) {
+      if (r == 0) return buf;
+      return absl::StrFormat("strerror_r(%d) failed: %d", err, r);
+    }
+    static std::string Run(char*, int, const char* r) { return r; }
+  };
+  char buf[256];
+  return Finish::Run(buf, err, strerror_r(err, buf, sizeof(buf)));
+}
+#endif  // !GPR_WINDOWS
+}  // namespace
+
 absl::Status grpc_os_error(const grpc_core::DebugLocation& location, int err,
                            const char* call_name) {
+  auto err_string = StrError(err);
   absl::Status s =
-      StatusCreate(absl::StatusCode::kUnknown, strerror(err), location, {});
+      StatusCreate(absl::StatusCode::kUnknown, err_string, location, {});
   grpc_core::StatusSetInt(&s, grpc_core::StatusIntProperty::kErrorNo, err);
   grpc_core::StatusSetStr(&s, grpc_core::StatusStrProperty::kOsError,
-                          strerror(err));
+                          err_string);
   grpc_core::StatusSetStr(&s, grpc_core::StatusStrProperty::kSyscall,
                           call_name);
   return s;
diff --git a/test/core/end2end/fixtures/h2_local_abstract_uds_percent_encoded.cc b/test/core/end2end/fixtures/h2_local_abstract_uds_percent_encoded.cc
index 02ffbedb4d0..cfcbb63ba0d 100644
--- a/test/core/end2end/fixtures/h2_local_abstract_uds_percent_encoded.cc
+++ b/test/core/end2end/fixtures/h2_local_abstract_uds_percent_encoded.cc
@@ -15,6 +15,7 @@
 #include <inttypes.h>
 #include <unistd.h>
 
+#include <atomic>
 #include <string>
 
 #include "absl/strings/str_format.h"
@@ -27,7 +28,7 @@
 #include "test/core/end2end/fixtures/local_util.h"
 #include "test/core/util/test_config.h"
 
-static int unique{0};
+static std::atomic<int> unique{0};
 
 static grpc_end2end_test_fixture chttp2_create_fixture_fullstack_uds(
     const grpc_channel_args* /*client_args*/,
diff --git a/test/core/end2end/fixtures/h2_local_uds.cc b/test/core/end2end/fixtures/h2_local_uds.cc
index 91756e18795..792b3bc2c9a 100644
--- a/test/core/end2end/fixtures/h2_local_uds.cc
+++ b/test/core/end2end/fixtures/h2_local_uds.cc
@@ -19,6 +19,7 @@
 #include <inttypes.h>
 #include <unistd.h>
 
+#include <atomic>
 #include <string>
 
 #include "absl/strings/str_format.h"
@@ -31,7 +32,7 @@
 #include "test/core/end2end/fixtures/local_util.h"
 #include "test/core/util/test_config.h"
 
-static int unique = 1;
+static std::atomic<int> unique{1};
 
 static grpc_end2end_test_fixture chttp2_create_fixture_fullstack_uds(
     const grpc_channel_args* /*client_args*/,
diff --git a/test/core/end2end/fixtures/h2_local_uds_percent_encoded.cc b/test/core/end2end/fixtures/h2_local_uds_percent_encoded.cc
index 495f4bb95a4..7b4a4e4eef8 100644
--- a/test/core/end2end/fixtures/h2_local_uds_percent_encoded.cc
+++ b/test/core/end2end/fixtures/h2_local_uds_percent_encoded.cc
@@ -19,6 +19,7 @@
 #include <inttypes.h>
 #include <unistd.h>
 
+#include <atomic>
 #include <string>
 
 #include "absl/strings/str_format.h"
@@ -31,7 +32,7 @@
 #include "test/core/end2end/fixtures/local_util.h"
 #include "test/core/util/test_config.h"
 
-static int unique = 1;
+static std::atomic<int> unique{1};
 
 static grpc_end2end_test_fixture chttp2_create_fixture_fullstack_uds(
     const grpc_channel_args* /*client_args*/,
diff --git a/test/core/end2end/fixtures/h2_uds.cc b/test/core/end2end/fixtures/h2_uds.cc
index 4c990cc0d55..0f17b78371c 100644
--- a/test/core/end2end/fixtures/h2_uds.cc
+++ b/test/core/end2end/fixtures/h2_uds.cc
@@ -20,6 +20,7 @@
 #include <string.h>
 #include <unistd.h>
 
+#include <atomic>
 #include <string>
 #include <utility>
 
@@ -37,7 +38,7 @@ struct fullstack_fixture_data {
   std::string localaddr;
 };
 
-static int unique = 1;
+static std::atomic<int> unique{1};
 
 static grpc_end2end_test_fixture chttp2_create_fixture_fullstack_base(
     std::string addr) {
diff --git a/test/core/end2end/fixtures/h2_uds_abstract.cc b/test/core/end2end/fixtures/h2_uds_abstract.cc
index a06efd5358c..9d22cd65a17 100644
--- a/test/core/end2end/fixtures/h2_uds_abstract.cc
+++ b/test/core/end2end/fixtures/h2_uds_abstract.cc
@@ -20,6 +20,7 @@
 #include <string.h>
 #include <unistd.h>
 
+#include <atomic>
 #include <string>
 #include <utility>
 
@@ -37,7 +38,7 @@ struct fullstack_fixture_data {
   std::string localaddr;
 };
 
-static int unique = 1;
+static std::atomic<int> unique{1};
 
 static grpc_end2end_test_fixture chttp2_create_fixture_fullstack_base(
     std::string addr) {
diff --git a/test/core/end2end/tests/hpack_size.cc b/test/core/end2end/tests/hpack_size.cc
index 6bea0ecfb1a..0bc80af2268 100644
--- a/test/core/end2end/tests/hpack_size.cc
+++ b/test/core/end2end/tests/hpack_size.cc
@@ -20,16 +20,22 @@
 #include <stdio.h>
 #include <string.h>
 
+#include <algorithm>
+#include <memory>
 #include <string>
+#include <vector>
 
 #include "absl/strings/str_format.h"
+#include "absl/synchronization/notification.h"
 
+#include <grpc/event_engine/event_engine.h>
 #include <grpc/grpc.h>
 #include <grpc/impl/codegen/propagation_bits.h>
 #include <grpc/slice.h>
 #include <grpc/status.h>
 #include <grpc/support/log.h>
 
+#include "src/core/lib/event_engine/default_event_engine.h"
 #include "src/core/lib/gpr/useful.h"
 #include "test/core/end2end/cq_verifier.h"
 #include "test/core/end2end/end2end_tests.h"
@@ -386,11 +392,21 @@ void hpack_size(grpc_end2end_test_config config) {
                                           1000, 32768, 4 * 1024 * 1024};
   size_t i, j;
 
+  auto event_engine = grpc_event_engine::experimental::GetDefaultEventEngine();
+  std::vector<std::shared_ptr<absl::Notification>> dones;
   for (i = 0; i < GPR_ARRAY_SIZE(interesting_sizes); i++) {
     for (j = 0; j < GPR_ARRAY_SIZE(interesting_sizes); j++) {
-      test_size(config, interesting_sizes[i], interesting_sizes[j]);
+      auto done = std::make_shared<absl::Notification>();
+      dones.push_back(done);
+      event_engine->Run([done, config, i, j] {
+        test_size(config, interesting_sizes[i], interesting_sizes[j]);
+        done->Notify();
+      });
     }
   }
+  for (auto& done : dones) {
+    done->WaitForNotification();
+  }
 }
 
 void hpack_size_pre_init(void) {}