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

[Rafael Avila de Espindola via llvm-commits]

NAKAMURA Takumi <geek4civic at gmail.com> writes:

> 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);


Can you try if you get a crash with just

------------------------------
#include <pthread.h>
#include <assert.h>
static void *worker_stub(void *) {
  return nullptr;
}
int main() {
  pthread_t th;
 auto r = pthread_create(&th, nullptr, worker_stub, (void*)42);
 (void)r;
 assert(r == 0);
 auto p = reinterpret_cast<volatile int*>(th);
 for (int j = 0; j < 1000 * 1000; ++j)
   *p;
 pthread_detach(th);
}
------------------------------

Cheers,
Rafael