Hello,

The refcounted basic_string implementation contains several data races
on _M_refcount:
1. _M_is_leaked loads _M_refcount concurrently with mutations of
_M_refcount. This loads needs to be memory_order_relaxed load, as
_M_is_leaked predicate does not change under the feet.
2. _M_is_shared loads _M_refcount concurrently with mutations of
_M_refcount. This loads needs to be memory_order_acquire, as another
thread can drop _M_refcount to zero concurrently which makes the
string non-shared and so the current thread can mutate the string. We
need reads of the string in another thread (while it was shared) to
happen-before the writes to the string in this thread (now that it is
non-shared).

This patch adds __gnu_cxx::__atomic_load_dispatch function to do the
loads of _M_refcount. The function does an acquire load. Acquire is
non needed for _M_is_leaked, but for simplicity as still do acquire
(hopefully the refcounted impl will go away in future).
This patch also uses the new function to do loads of _M_refcount in
string implementation.

I did non update doc/xml/manual/concurrency_extensions.xml to document
__gnu_cxx::__atomic_load_dispatch, because I am not sure we want to
extend that api now that we have language-level atomics. If you still
want me to update it, please say how to regenerate
doc/html/manual/ext_concurrency.html.

The race was detected with ThreadSanitizer on the following program:

#define _GLIBCXX_USE_CXX11_ABI 0
#include <string>
#include <thread>
#include <iostream>
#include <chrono>

int main() {
  std::string s = "foo";
  std::thread t([=](){
    std::string x = s;
    std::cout << &x << std::endl;
  });
  std::this_thread::sleep_for(std::chrono::seconds(1));
  std::cout << &s[0] << std::endl;
  t.join();
}

$ g++ string.cc -std=c++11 -pthread -O1 -g -fsanitize=thread
$ ./a.out

WARNING: ThreadSanitizer: data race (pid=98135)
  Read of size 4 at 0x7d080000efd0 by main thread:
    #0 std::string::_Rep::_M_is_leaked() const
include/c++/6.0.0/bits/basic_string.h:2605 (a.out+0x000000401d7c)
    #1 std::string::_M_leak()
include/c++/6.0.0/bits/basic_string.h:2730 (a.out+0x000000401d7c)
    #2 std::string::operator[](unsigned long)
include/c++/6.0.0/bits/basic_string.h:3274 (a.out+0x000000401d7c)
    #3 main /tmp/string.cc:14 (a.out+0x000000401d7c)

  Previous atomic write of size 4 at 0x7d080000efd0 by thread T1:
    #0 __tsan_atomic32_fetch_add
../../../../libsanitizer/tsan/tsan_interface_atomic.cc:611
(libtsan.so.0+0x00000005811f)
    #1 __exchange_and_add include/c++/6.0.0/ext/atomicity.h:59
(a.out+0x000000401a19)
    #2 __exchange_and_add_dispatch
include/c++/6.0.0/ext/atomicity.h:92 (a.out+0x000000401a19)
    #3 std::string::_Rep::_M_dispose(std::allocator<char> const&)
include/c++/6.0.0/bits/basic_string.h:2659 (a.out+0x000000401a19)
    #4 std::basic_string<char, std::char_traits<char>,
std::allocator<char> >::~basic_string()
include/c++/6.0.0/bits/basic_string.h:2961 (a.out+0x000000401a19)
    #5 ~<lambda> /tmp/string.cc:9 (a.out+0x000000401a19)
    #6 ~_Head_base include/c++/6.0.0/tuple:102 (a.out+0x000000401a19)
    #7 ~_Tuple_impl include/c++/6.0.0/tuple:338 (a.out+0x000000401a19)
    #8 ~tuple include/c++/6.0.0/tuple:521 (a.out+0x000000401a19)
    #9 ~_Bind_simple include/c++/6.0.0/functional:1503 (a.out+0x000000401a19)
    #10 ~_Impl include/c++/6.0.0/thread:107 (a.out+0x000000401a19)
    #11 destroy<std::thread::_Impl<std::_Bind_simple<main()::<lambda()>()>
> > include/c++/6.0.0/ext/new_allocator.h:124 (a.out+0x0000004015c7)
    #12 _S_destroy<std::allocator<std::thread::_Impl<std::_Bind_simple<main()::<lambda()>()>
> >, std::thread::_Impl<std::_Bind_simple<main()::<lambda()>()> > >
include/c++/6.0.0/bits/alloc_traits.h:236 (a.out+0x0000004015c7)
    #13 destroy<std::thread::_Impl<std::_Bind_simple<main()::<lambda()>()>
> > include/c++/6.0.0/bits/alloc_traits.h:336 (a.out+0x0000004015c7)
    #14 _M_dispose include/c++/6.0.0/bits/shared_ptr_base.h:529
(a.out+0x0000004015c7)
    #15 std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release()
/usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/shared_ptr_base.h:150
(libstdc++.so.6+0x0000000b6da4)
    #16 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count()
/usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/shared_ptr_base.h:662
(libstdc++.so.6+0x0000000b6da4)
    #17 std::__shared_ptr<std::thread::_Impl_base,
(__gnu_cxx::_Lock_policy)2>::~__shared_ptr()
/usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/shared_ptr_base.h:928
(libstdc++.so.6+0x0000000b6da4)
    #18 std::shared_ptr<std::thread::_Impl_base>::~shared_ptr()
/usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/shared_ptr.h:93
(libstdc++.so.6+0x0000000b6da4)
    #19 execute_native_thread_routine
libstdc++-v3/src/c++11/thread.cc:79 (libstdc++.so.6+0x0000000b6da4)

  Location is heap block of size 28 at 0x7d080000efc0 allocated by main thread:
    #0 operator new(unsigned long)
../../../../libsanitizer/tsan/tsan_interceptors.cc:571
(libtsan.so.0+0x0000000255a3)
    #1 __gnu_cxx::new_allocator<char>::allocate(unsigned long, void
const*) /usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/ext/new_allocator.h:104
(libstdc++.so.6+0x0000000cbaa8)
    #2 std::string::_Rep::_S_create(unsigned long, unsigned long,
std::allocator<char> const&)
/usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.tcc:1051
(libstdc++.so.6+0x0000000cbaa8)
    #3 __libc_start_main <null> (libc.so.6+0x000000021ec4)

  Thread T1 (tid=98137, finished) created by main thread at:
    #0 pthread_create
../../../../libsanitizer/tsan/tsan_interceptors.cc:895
(libtsan.so.0+0x000000026d04)
    #1 __gthread_create
/usr/local/google/home/dvyukov/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/gthr-default.h:662
(libstdc++.so.6+0x0000000b6e52)
    #2 std::thread::_M_start_thread(std::shared_ptr<std::thread::_Impl_base>,
void (*)()) libstdc++-v3/src/c++11/thread.cc:149
(libstdc++.so.6+0x0000000b6e52)
    #3 __libc_start_main <null> (libc.so.6+0x000000021ec4)

OK for trunk?