diff --git a/base/include/android-base/logging.h b/base/include/android-base/logging.h
index 7fd0182..cc162cd 100644
--- a/base/include/android-base/logging.h
+++ b/base/include/android-base/logging.h
@@ -115,7 +115,6 @@
 
 void DefaultAborter(const char* abort_message);
 
-std::string GetDefaultTag();
 void SetDefaultTag(const std::string& tag);
 
 // We expose this even though it is the default because a user that wants to
diff --git a/base/liblog_symbols.cpp b/base/liblog_symbols.cpp
index d0e2eaa..d5dfcd2 100644
--- a/base/liblog_symbols.cpp
+++ b/base/liblog_symbols.cpp
@@ -50,6 +50,7 @@
     DLSYM(__android_log_default_aborter)
     DLSYM(__android_log_set_minimum_priority);
     DLSYM(__android_log_get_minimum_priority);
+    DLSYM(__android_log_set_default_tag);
 #undef DLSYM
 
     return real_liblog_functions;
@@ -72,6 +73,7 @@
         .__android_log_default_aborter = __android_log_default_aborter,
         .__android_log_set_minimum_priority = __android_log_set_minimum_priority,
         .__android_log_get_minimum_priority = __android_log_get_minimum_priority,
+        .__android_log_set_default_tag = __android_log_set_default_tag,
     };
   }();
   return liblog_functions;
diff --git a/base/liblog_symbols.h b/base/liblog_symbols.h
index c68fff6..d3134e9 100644
--- a/base/liblog_symbols.h
+++ b/base/liblog_symbols.h
@@ -38,6 +38,7 @@
   void (*__android_log_default_aborter)(const char* abort_message);
   int (*__android_log_set_minimum_priority)(int priority);
   int (*__android_log_get_minimum_priority)();
+  void (*__android_log_set_default_tag)(const char* tag);
 };
 
 const std::optional<LibLogFunctions>& GetLibLogFunctions();
diff --git a/base/logging.cpp b/base/logging.cpp
index 508871d..1d8ef57 100644
--- a/base/logging.cpp
+++ b/base/logging.cpp
@@ -211,26 +211,27 @@
   return aborter;
 }
 
+// Only used for Q fallback.
 static std::recursive_mutex& TagLock() {
   static auto& tag_lock = *new std::recursive_mutex();
   return tag_lock;
 }
+// Only used for Q fallback.
 static std::string* gDefaultTag;
-std::string GetDefaultTag() {
-  std::lock_guard<std::recursive_mutex> lock(TagLock());
-  if (gDefaultTag == nullptr) {
-    return "";
-  }
-  return *gDefaultTag;
-}
+
 void SetDefaultTag(const std::string& tag) {
-  std::lock_guard<std::recursive_mutex> lock(TagLock());
-  if (gDefaultTag != nullptr) {
-    delete gDefaultTag;
-    gDefaultTag = nullptr;
-  }
-  if (!tag.empty()) {
-    gDefaultTag = new std::string(tag);
+  static auto& liblog_functions = GetLibLogFunctions();
+  if (liblog_functions) {
+    liblog_functions->__android_log_set_default_tag(tag.c_str());
+  } else {
+    std::lock_guard<std::recursive_mutex> lock(TagLock());
+    if (gDefaultTag != nullptr) {
+      delete gDefaultTag;
+      gDefaultTag = nullptr;
+    }
+    if (!tag.empty()) {
+      gDefaultTag = new std::string(tag);
+    }
   }
 }
 
@@ -574,24 +575,18 @@
                          const char* message) {
   static auto& liblog_functions = GetLibLogFunctions();
   auto priority = LogSeverityToPriority(severity);
-  if (tag == nullptr) {
-    std::lock_guard<std::recursive_mutex> lock(TagLock());
-    if (gDefaultTag == nullptr) {
-      gDefaultTag = new std::string(getprogname());
-    }
-
-    if (liblog_functions) {
-      __android_logger_data logger_data = {sizeof(__android_logger_data), LOG_ID_DEFAULT, priority,
-                                           gDefaultTag->c_str(),          file,           line};
-      __android_log_write_logger_data(&logger_data, message);
-    } else {
-      Logger()(DEFAULT, severity, gDefaultTag->c_str(), file, line, message);
-    }
+  if (liblog_functions) {
+    __android_logger_data logger_data = {
+        sizeof(__android_logger_data), LOG_ID_DEFAULT, priority, tag, file, line};
+    __android_log_write_logger_data(&logger_data, message);
   } else {
-    if (liblog_functions) {
-      __android_logger_data logger_data = {
-          sizeof(__android_logger_data), LOG_ID_DEFAULT, priority, tag, file, line};
-      __android_log_write_logger_data(&logger_data, message);
+    if (tag == nullptr) {
+      std::lock_guard<std::recursive_mutex> lock(TagLock());
+      if (gDefaultTag == nullptr) {
+        gDefaultTag = new std::string(getprogname());
+      }
+
+      Logger()(DEFAULT, severity, gDefaultTag->c_str(), file, line, message);
     } else {
       Logger()(DEFAULT, severity, tag, file, line, message);
     }
diff --git a/base/logging_test.cpp b/base/logging_test.cpp
index 8f46196..3a453e6 100644
--- a/base/logging_test.cpp
+++ b/base/logging_test.cpp
@@ -603,21 +603,6 @@
   LOG(ERROR) << "foobar";
 }
 
-TEST(logging, SetDefaultTag) {
-  constexpr const char* expected_tag = "test_tag";
-  constexpr const char* expected_msg = "foobar";
-  CapturedStderr cap;
-  {
-    std::string old_default_tag = android::base::GetDefaultTag();
-    android::base::SetDefaultTag(expected_tag);
-    android::base::ScopedLogSeverity sls(android::base::LogSeverity::INFO);
-    LOG(INFO) << expected_msg;
-    android::base::SetDefaultTag(old_default_tag);
-  }
-  ASSERT_NO_FATAL_FAILURE(
-      CheckMessage(cap, android::base::LogSeverity::INFO, expected_msg, expected_tag));
-}
-
 TEST(logging, StdioLogger) {
   CapturedStderr cap_err;
   CapturedStdout cap_out;
diff --git a/liblog/include/android/log.h b/liblog/include/android/log.h
index 00aafaa..c84ddf7 100644
--- a/liblog/include/android/log.h
+++ b/liblog/include/android/log.h
@@ -276,6 +276,13 @@
  */
 int __android_log_get_minimum_priority();
 
+/**
+ * Sets the default tag if no tag is provided when writing a log message.  Defaults to
+ * getprogname().  This truncates tag to the maximum log message size, though appropriate tags
+ * should be much smaller.
+ */
+void __android_log_set_default_tag(const char* tag);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/liblog/liblog.map.txt b/liblog/liblog.map.txt
index 62b9805..198cdae 100644
--- a/liblog/liblog.map.txt
+++ b/liblog/liblog.map.txt
@@ -73,6 +73,7 @@
     __android_log_logd_logger;
     __android_log_security_bswrite; # apex
     __android_log_set_aborter;
+    __android_log_set_default_tag;
     __android_log_set_logger;
     __android_log_set_minimum_priority;
     __android_log_stderr_logger;
diff --git a/liblog/logger_write.cpp b/liblog/logger_write.cpp
index 1783854..e86d9ec 100644
--- a/liblog/logger_write.cpp
+++ b/liblog/logger_write.cpp
@@ -16,6 +16,7 @@
 
 #include <errno.h>
 #include <inttypes.h>
+#include <libgen.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/time.h>
@@ -31,6 +32,7 @@
 #include <private/android_logger.h>
 
 #include "android/log.h"
+#include "log/log_read.h"
 #include "logger.h"
 #include "rwlock.h"
 #include "uio.h"
@@ -106,6 +108,45 @@
 #endif
 }
 
+#if defined(__GLIBC__) || defined(_WIN32)
+static const char* getprogname() {
+#if defined(__GLIBC__)
+  return program_invocation_short_name;
+#elif defined(_WIN32)
+  static bool first = true;
+  static char progname[MAX_PATH] = {};
+
+  if (first) {
+    char path[PATH_MAX + 1];
+    DWORD result = GetModuleFileName(nullptr, path, sizeof(path) - 1);
+    if (result == 0 || result == sizeof(path) - 1) return "";
+    path[PATH_MAX - 1] = 0;
+
+    char* path_basename = basename(path);
+
+    snprintf(progname, sizeof(progname), "%s", path_basename);
+    first = false;
+  }
+
+  return progname;
+#endif
+}
+#endif
+
+// It's possible for logging to happen during static initialization before our globals are
+// initialized, so we place this std::string in a function such that it is initialized on the first
+// call.
+static std::string& GetDefaultTag() {
+  static std::string default_tag = getprogname();
+  return default_tag;
+}
+static RwLock default_tag_lock;
+
+void __android_log_set_default_tag(const char* tag) {
+  auto lock = std::unique_lock{default_tag_lock};
+  GetDefaultTag().assign(tag, 0, LOGGER_ENTRY_MAX_PAYLOAD);
+}
+
 static int minimum_log_priority = ANDROID_LOG_DEFAULT;
 int __android_log_set_minimum_priority(int priority) {
   int old_minimum_log_priority = minimum_log_priority;
@@ -270,7 +311,11 @@
 }
 
 void __android_log_write_logger_data(__android_logger_data* logger_data, const char* msg) {
-  if (logger_data->tag == nullptr) logger_data->tag = "";
+  auto tag_lock = std::shared_lock{default_tag_lock, std::defer_lock};
+  if (logger_data->tag == nullptr) {
+    tag_lock.lock();
+    logger_data->tag = GetDefaultTag().c_str();
+  }
 
 #if __BIONIC__
   if (logger_data->priority == ANDROID_LOG_FATAL) {
diff --git a/liblog/tests/Android.bp b/liblog/tests/Android.bp
index 3288954..b3364f9 100644
--- a/liblog/tests/Android.bp
+++ b/liblog/tests/Android.bp
@@ -54,6 +54,7 @@
     ],
     srcs: [
         "libc_test.cpp",
+        "liblog_default_tag.cpp",
         "liblog_global_state.cpp",
         "liblog_test.cpp",
         "log_id_test.cpp",
diff --git a/liblog/tests/liblog_default_tag.cpp b/liblog/tests/liblog_default_tag.cpp
new file mode 100644
index 0000000..a5baa9f
--- /dev/null
+++ b/liblog/tests/liblog_default_tag.cpp
@@ -0,0 +1,99 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// LOG_TAG must be unset for android-base's logging to use a default tag.
+#undef LOG_TAG
+
+#include <stdlib.h>
+
+#include <android-base/file.h>
+#include <android-base/logging.h>
+#include <android-base/properties.h>
+#include <android/log.h>
+
+#include <gtest/gtest.h>
+
+TEST(liblog_default_tag, no_default_tag_libbase_write_first) {
+  using namespace android::base;
+  bool message_seen = false;
+  std::string expected_tag = "";
+  SetLogger([&](LogId, LogSeverity, const char* tag, const char*, unsigned int, const char*) {
+    message_seen = true;
+    EXPECT_EQ(expected_tag, tag);
+  });
+
+  expected_tag = getprogname();
+  LOG(WARNING) << "message";
+  EXPECT_TRUE(message_seen);
+  message_seen = false;
+
+  __android_log_buf_write(LOG_ID_MAIN, ANDROID_LOG_WARN, nullptr, "message");
+  EXPECT_TRUE(message_seen);
+}
+
+TEST(liblog_default_tag, no_default_tag_liblog_write_first) {
+  using namespace android::base;
+  bool message_seen = false;
+  std::string expected_tag = "";
+  SetLogger([&](LogId, LogSeverity, const char* tag, const char*, unsigned int, const char*) {
+    message_seen = true;
+    EXPECT_EQ(expected_tag, tag);
+  });
+
+  expected_tag = getprogname();
+  __android_log_buf_write(LOG_ID_MAIN, ANDROID_LOG_WARN, nullptr, "message");
+  EXPECT_TRUE(message_seen);
+  message_seen = false;
+
+  LOG(WARNING) << "message";
+  EXPECT_TRUE(message_seen);
+}
+
+TEST(liblog_default_tag, libbase_sets_default_tag) {
+  using namespace android::base;
+  bool message_seen = false;
+  std::string expected_tag = "libbase_test_tag";
+  SetLogger([&](LogId, LogSeverity, const char* tag, const char*, unsigned int, const char*) {
+    message_seen = true;
+    EXPECT_EQ(expected_tag, tag);
+  });
+  SetDefaultTag(expected_tag);
+
+  __android_log_buf_write(LOG_ID_MAIN, ANDROID_LOG_WARN, nullptr, "message");
+  EXPECT_TRUE(message_seen);
+  message_seen = false;
+
+  LOG(WARNING) << "message";
+  EXPECT_TRUE(message_seen);
+}
+
+TEST(liblog_default_tag, liblog_sets_default_tag) {
+  using namespace android::base;
+  bool message_seen = false;
+  std::string expected_tag = "liblog_test_tag";
+  SetLogger([&](LogId, LogSeverity, const char* tag, const char*, unsigned int, const char*) {
+    message_seen = true;
+    EXPECT_EQ(expected_tag, tag);
+  });
+  __android_log_set_default_tag(expected_tag.c_str());
+
+  __android_log_buf_write(LOG_ID_MAIN, ANDROID_LOG_WARN, nullptr, "message");
+  EXPECT_TRUE(message_seen);
+  message_seen = false;
+
+  LOG(WARNING) << "message";
+  EXPECT_TRUE(message_seen);
+}
\ No newline at end of file