1054335 - Shutdown deadlock with profiler in RemoveActiveSampler

Status: RESOLVED FIXED

(Core :: Gecko Profiler, defect)

Description

[Markus Stange [:mstange]]

I've just hit this in a local build which won't shut down because the main thread is blocked in SamplerThread::RemoveActiveSampler. It's waiting for the sampler thread to join, on the line instance_->Join(); below. Unfortunately, RemoveActiveSampler holds the sRegisteredThreadMutex:

  static void RemoveActiveSampler(Sampler* sampler) {
    mozilla::MutexAutoLock lock(*Sampler::sRegisteredThreadsMutex);
    instance_->Join();

But the sampler thread is currently blocking on this mutex:

  virtual void Run() {
    while (SamplerRegistry::sampler->IsActive()) {
      if (!SamplerRegistry::sampler->IsPaused()) {
        mozilla::MutexAutoLock lock(*Sampler::sRegisteredThreadsMutex); // <--
        std::vector<ThreadInfo*> threads =
          SamplerRegistry::sampler->GetRegisteredThreads();
        bool isFirstProfiledThread = true;

And can't finish.

Comment 2

[Gerald Squelart (he/him) (not at Mozilla since 2022-09-15)]

Thread registration has changed a lot since then.
And I've verified that the SamplerThread's thread is only joined when no locks are held, so this deadlock should not happen.
So I'll call this bug fixed.