choller

int32_t oldVals = myVal.bitfields;            // load all bitfields
int32_t newVals = oldVals | 1 << fieldOffset; // modify just one
myVal.bitfields = newVals;                    // write all bitfields

choller

static bool sInitialized = false;
static Mutex sMutex; // guards sInitialized

// racily check if initialized (gotta go fast!)
if (!sInitialized) {

  // now properly check
  auto lock = sMutex.Lock();

  if (!sInitialized) {

choller

WARNING: ThreadSanitizer: data race (pid=1544)
  Write of size 8 at 0x7b30000048d8 by thread T21:
    #0 __xstat64 compiler-rt/lib/tsan/../sanitizer_common/sanitizer_common_interceptors.inc:6607:3
    #1 stat64 /usr/include/x86_64-linux-gnu/sys/stat.h:504:10
    #2 FillStatCache xpcom/io/nsLocalFileUnix.cpp:304:7
    #3 nsLocalFile::GetPermissions(unsigned int*) xpcom/io/nsLocalFileUnix.cpp:1254:3
    #4 nsAtomicFileOutputStream::DoOpen() netwerk/base/nsFileStreams.cpp:804:9
    [...]
    #10 mozilla::PreferencesWriter::Write([...]) modules/libpref/Preferences.cpp:2907:10
    [...]

choller

WARNING: ThreadSanitizer: data race (pid=1121)
  Read of size 2 at 0x7b74001a6fdc by main thread (mutexes: write M899729694995224136, write M119199653296761384):
    #0 mozilla::layers::AsyncPanZoomController::ApplyAsyncTestAttributes() gfx/layers/apz/src/AsyncPanZoomController.cpp:4218:7
    #1 AutoApplyAsyncTestAttributes gfx/layers/apz/src/AsyncPanZoomController.cpp:596:10
    [...]
    #34 main browser/app/nsBrowserApp.cpp:308:16

  Previous write of size 2 at 0x7b74001a6fdc by thread T29:
    #0 MarkAsyncTransformAppliedToContent gfx/layers/apz/src/AsyncPanZoomController.h:1605:37
    #1 mozilla::layers::APZSampler::MarkAsyncTransformAppliedToContent([...]) gfx/layers/apz/src/APZSampler.cpp:192:21

choller

#include <iostream>
#include <thread>

int x = 0;

void print_x() {
  std::cerr << "x is " << x << std::endl;
}

void set_x(int new_x) {