std::thread::detach() rarely crashes in Parallel.cpp

[NAKAMURA Takumi via llvm-commits]

It was discussed in https://reviews.llvm.org/D39038

I suffered when check-lld behaves flaky. For example;
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/19673

I have been investigating and I guess it triggered bug or something in
libpthread or Linux kernel.
pthread_detach() in std::thread::detach() is crashing with segv.
Seems pthread (internal struct in Linux nptl) is inaccessible.
The crash is reproducible w/o std::thread, but with libpthread.
In my environment, crash can be seen several times per million attempt.

This is my testcase. (in Parallel.cpp)

        pthread_t th;
        auto r = pthread_create(&th, nullptr, worker_stub, (void*)this);
        (void)r;
        assert(r == 0);
        auto p = reinterpret_cast<volatile int*>(th);
        for (int j = 0; j < 1000 * 1000; ++j)
          *p;
        pthread_detach(th);

Accessing *p sometimes raises segv.

Ideally, Linux (or distro) should be fixed, but I suppose the distro,
Ubuntu 14.04 is widely used.
I would like to propose we could avoid std::thread().detach()

As far as I tried, this successfully runs to me with million of iterations.
- Use naked pthread_create()
  a) Create the thread with PTHREAD_CREATE_DETACHED
  b) Do pthread_detach(pthread_self()) in the worker thread.

Note, (b) doesn't work with std::thread.

Takumi
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