Looks like the issue is that content::SiteIsolationStatsGathererBrowserTest::SetUpCommandLine() creates a thread. This happens before content::ContentBrowserTest::SetUp() calls content::BrowserTestBase::SetUp() which ends up initializing logging. Therefore logging operation on the created thread are racy.

Creating a thread from SetUpCommandLine() sounds wrong (should be in SetUp() after calling the parent's SetUp()?). And if we can't help it, then we should call content::BrowserTestBase::SetUp() as the first thing in content::ContentBrowserTest::SetUp() (setting up the parent before self is also a common paradigm).

Closest blame I can find is a year old... https://codereview.chromium.org/1411073005 (@svaldez)

Use
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_tsan_rel_ng
to see these errors in try bots.

Example TSAN log:

SecurityExploitBrowserTest.AttemptDuplicateRenderViewHost (run #1):
[ RUN      ] SecurityExploitBrowserTest.AttemptDuplicateRenderViewHost
==================

WARNING: ThreadSanitizer: data race (pid=28554)
  Read of size 4 at 0x00000a778da4 by thread T1:
    #0 logging::ShouldCreateLogMessage(int) base/logging.cc:417:10 (content_browsertests+0x00000305cc87)
    #1 operator-> base/memory/ref_counted.h:322:5 (content_browsertests+0x00000306f2d8)
    #2 ReloadWorkQueue base/message_loop/message_loop.cc:478 (content_browsertests+0x00000306f2d8)
    #3 base::MessageLoop::DoWork() base/message_loop/message_loop.cc:499 (content_browsertests+0x00000306f2d8)
    #4 base::MessagePumpLibevent::Run(base::MessagePump::Delegate*) base/message_loop/message_pump_libevent.cc:218:31 (content_browsertests+0x000003075e60)
    #5 base::MessageLoop::RunHandler() base/message_loop/message_loop.cc:378:10 (content_browsertests+0x00000306df72)
    #6 base::RunLoop::Run() base/run_loop.cc:37:10 (content_browsertests+0x0000030a9b0e)
    #7 base::Thread::Run(base::RunLoop*) base/threading/thread.cc:245:13 (content_browsertests+0x0000030e86ab)
    #8 base::Thread::ThreadMain() base/threading/thread.cc:328:3 (content_browsertests+0x0000030e8db6)
    #9 base::(anonymous namespace)::ThreadFunc(void*) base/threading/platform_thread_posix.cc:71:13 (content_browsertests+0x0000030de6b8)

  Previous write of size 4 at 0x00000a778da4 by main thread:
    #0 logging::BaseInitLoggingImpl_built_with_NDEBUG(logging::LoggingSettings const&) base/logging.cc:378:25 (content_browsertests+0x00000305c934)
    #1 InitLogging base/logging.h:232:10 (content_browsertests+0x000002ae705d)
    #2 InitLogging content/shell/app/shell_main_delegate.cc:111 (content_browsertests+0x000002ae705d)
    #3 content::ShellMainDelegate::BasicStartupComplete(int*) content/shell/app/shell_main_delegate.cc:145 (content_browsertests+0x000002ae705d)
    #4 content::ContentMainRunnerImpl::Initialize(content::ContentMainParams const&) content/app/content_main_runner.cc:558:33 (content_browsertests+0x0000020c6c49)
    #5 content::ContentMain(content::ContentMainParams const&) content/app/content_main.cc:16:32 (content_browsertests+0x0000020bca1e)
    #6 content::BrowserTestBase::SetUp() content/public/test/browser_test_base.cc:316:3 (content_browsertests+0x000002a7d6ab)
    #7 content::ContentBrowserTest::SetUp() content/public/test/content_browser_test.cc:94:20 (content_browsertests+0x000002a76b66)
    #8 HandleExceptionsInMethodIfSupported<testing::Test, void> testing/gtest/src/gtest.cc:2458:12 (content_browsertests+0x000002d31f30)
    #9 testing::Test::Run() testing/gtest/src/gtest.cc:2470 (content_browsertests+0x000002d31f30)
    #10 testing::TestInfo::Run() testing/gtest/src/gtest.cc:2656:11 (content_browsertests+0x000002d332fc)
    #11 testing::TestCase::Run() testing/gtest/src/gtest.cc:2774:28 (content_browsertests+0x000002d33bd6)
    #12 testing::internal::UnitTestImpl::RunAllTests() testing/gtest/src/gtest.cc:4647:43 (content_browsertests+0x000002d3eae6)
    #13 HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> testing/gtest/src/gtest.cc:2458:12 (content_browsertests+0x000002d3e379)
    #14 testing::UnitTest::Run() testing/gtest/src/gtest.cc:4255 (content_browsertests+0x000002d3e379)
    #15 RUN_ALL_TESTS testing/gtest/include/gtest/gtest.h:2237:46 (content_browsertests+0x000002ab47c9)
    #16 base::TestSuite::Run() base/test/test_suite.cc:271 (content_browsertests+0x000002ab47c9)
    #17 content::ContentTestLauncherDelegate::RunTestSuite(int, char**) content/test/content_test_launcher.cc:105:48 (content_browsertests+0x000002a7c70b)
    #18 content::LaunchTests(content::TestLauncherDelegate*, int, int, char**) content/public/test/test_launcher.cc:520:31 (content_browsertests+0x000002a9e663)
    #19 main content/test/content_test_launcher.cc:131:10 (content_browsertests+0x000002a7c692)

  Location is global 'logging::(anonymous namespace)::g_logging_destination' of size 4 at 0x00000a778da4 (content_browsertests+0x00000a778da4)

  Thread T1 'EmbeddedTestServer IO Thread' (tid=28566, running) created by main thread at:
    #0 pthread_create <null> (content_browsertests+0x0000004b4d75)
    #1 base::(anonymous namespace)::CreateThread(unsigned long, bool, base::PlatformThread::Delegate*, base::PlatformThreadHandle*, base::ThreadPriority) base/threading/platform_thread_posix.cc:110:13 (content_browsertests+0x0000030de166)
    #2 base::PlatformThread::CreateWithPriority(unsigned long, base::PlatformThread::Delegate*, base::PlatformThreadHandle*, base::ThreadPriority) base/threading/platform_thread_posix.cc:191:10 (content_browsertests+0x0000030de025)
    #3 base::Thread::StartWithOptions(base::Thread::Options const&) base/threading/thread.cc:112:15 (content_browsertests+0x0000030e7d83)
    #4 net::test_server::EmbeddedTestServer::StartAcceptingConnections() net/test/embedded_test_server/embedded_test_server.cc:172:3 (content_browsertests+0x000002c11e64)
    #5 net::test_server::EmbeddedTestServer::Start() net/test/embedded_test_server/embedded_test_server.cc:89:3 (content_browsertests+0x000002c11645)
    #6 content::SecurityExploitBrowserTest::SetUpCommandLine(base::CommandLine*) content/browser/security_exploit_browsertest.cc:183:5 (content_browsertests+0x00000087a8b4)
    #7 content::ContentBrowserTest::SetUp() content/public/test/content_browser_test.cc:61:3 (content_browsertests+0x000002a76b59)
    #8 HandleExceptionsInMethodIfSupported<testing::Test, void> testing/gtest/src/gtest.cc:2458:12 (content_browsertests+0x000002d31f30)
    #9 testing::Test::Run() testing/gtest/src/gtest.cc:2470 (content_browsertests+0x000002d31f30)
    #10 testing::TestInfo::Run() testing/gtest/src/gtest.cc:2656:11 (content_browsertests+0x000002d332fc)
    #11 testing::TestCase::Run() testing/gtest/src/gtest.cc:2774:28 (content_browsertests+0x000002d33bd6)
    #12 testing::internal::UnitTestImpl::RunAllTests() testing/gtest/src/gtest.cc:4647:43 (content_browsertests+0x000002d3eae6)
    #13 HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> testing/gtest/src/gtest.cc:2458:12 (content_browsertests+0x000002d3e379)
    #14 testing::UnitTest::Run() testing/gtest/src/gtest.cc:4255 (content_browsertests+0x000002d3e379)
    #15 RUN_ALL_TESTS testing/gtest/include/gtest/gtest.h:2237:46 (content_browsertests+0x000002ab47c9)
    #16 base::TestSuite::Run() base/test/test_suite.cc:271 (content_browsertests+0x000002ab47c9)
    #17 content::ContentTestLauncherDelegate::RunTestSuite(int, char**) content/test/content_test_launcher.cc:105:48 (content_browsertests+0x000002a7c70b)
    #18 content::LaunchTests(content::TestLauncherDelegate*, int, int, char**) content/public/test/test_launcher.cc:520:31 (content_browsertests+0x000002a9e663)
    #19 main content/test/content_test_launcher.cc:131:10 (content_browsertests+0x000002a7c692)

SUMMARY: ThreadSanitizer: data race base/logging.cc:417:10 in logging::ShouldCreateLogMessage(int)