[Bug 1844022] Re: pthread_setattr_default_np race condition with pthread_create.

[Florian Weimer]

I assume the code in another_thread attempts to obtain the size of the
current thread.  But the code just reads back the default stack size,
which may have changed due to the set_default_thread_stack_size call.

One way to obtain the size of the current thread stack is this call:

      if(int err = ::pthread_getattr_np(pthread_self(), &attr))
                throw std::system_error(err, std::system_category(), "pthread_getattr_np");

This should give consistent results all the time.

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Title:
  pthread_setattr_default_np race condition with pthread_create.

Status in glibc package in Ubuntu:
  New

Bug description:
  pthread_setattr_default_np race condition with pthread_create.

  When creating a new thread with pthread_create and a NULL
  pthread_attr_t* argument, the pthread_attr_t* argument must come from
  the last call to pthread_setattr_default_np preceding the call to
  pthread_create.

  It seems that there is a race condition in glibc implementation such
  that pthread_attr_t* argument comes from a pthread_setattr_default_np
  call that succeeds a pthread_create call.

  Example:

  [max at supernova:~/src/test] $ g++ --version
  g++ (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0
  Copyright (C) 2018 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

  [max at supernova:~/src/test] $ cat test.cc
  #include <cstdio>
  #include <thread>
  #include <system_error>
  #include <pthread.h>

  void set_default_thread_stack_size(size_t size) {
      pthread_attr_t attr;
      if(int err = ::pthread_attr_init(&attr))
          throw std::system_error(err, std::system_category(), "pthread_attr_init");
      if(int err = ::pthread_attr_setstacksize(&attr, size))
          throw std::system_error(err, std::system_category(), "pthread_attr_setstacksize");
      if(int err = ::pthread_setattr_default_np(&attr))
          throw std::system_error(err, std::system_category(), "pthread_setattr_default_np");
      if(int err = ::pthread_attr_destroy(&attr))
          throw std::system_error(err, std::system_category(), "pthread_attr_destroy");
  }

  void another_thread() {
      pthread_attr_t attr;
      if(int err = ::pthread_attr_init(&attr))
          throw std::system_error(err, std::system_category(), "pthread_attr_init");
      size_t stack_size;
      if(int err = ::pthread_attr_getstacksize(&attr, &stack_size))
          throw std::system_error(err, std::system_category(), "pthread_attr_getstacksize");
      if(int err = ::pthread_attr_destroy(&attr))
          throw std::system_error(err, std::system_category(), "pthread_attr_destroy");
      std::printf("thread stack size is %zu\n", stack_size);
  }

  int main() {
      another_thread();
      std::thread t0(another_thread);
      set_default_thread_stack_size(1024 * 1024);
      std::thread t1(another_thread);
      t0.join();
      t1.join();
  }

  [max at supernova:~/src/test] $ g++ -std=gnu++14 -W{all,extra,error} -O3
  -march=native -o test test.cc -pthread

  [max at supernova:~/src/test] $ ldd test
   linux-vdso.so.1 (0x00007ffd74d74000)
   libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f597ed00000)
   libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f597eae8000)
   libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f597e8c9000)
   libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f597e4d8000)
   libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f597e13a000)
   /lib64/ld-linux-x86-64.so.2 (0x00007f597f28d000)

  [max at supernova:~/src/test] $ /lib/x86_64-linux-gnu/libc.so.6
  GNU C Library (Ubuntu GLIBC 2.27-3ubuntu1) stable release version 2.27.
  Copyright (C) 2018 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.
  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
  PARTICULAR PURPOSE.
  Compiled by GNU CC version 7.3.0.
  libc ABIs: UNIQUE IFUNC
  For bug reporting instructions, please see:
  <https://bugs.launchpad.net/ubuntu/+source/glibc/+bugs>.

  [max at supernova:~/src/test] $ ./test
  thread stack size is 8720384
  thread stack size is 1048576
  thread stack size is 1048576

  The above output is incorrect. The expected output is:

  thread stack size is 8720384
  thread stack size is 8720384
  thread stack size is 1048576

  If `t0.join();` line is moved to right after `std::thread
  t0(another_thread);` line then the code produces the correct output.

  It appears that setting the default pthread_attr_t is not
  atomic/ordered with respect to pthread_create calls.

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