libstdc++-v3/libsupc++/guard.cc

// Copyright (C) 2002-2021 Free Software Foundation, Inc.
//  
// This file is part of GCC.
//
// GCC is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3, or (at your option)
// any later version.

// GCC is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

// Written by Mark Mitchell, CodeSourcery LLC, <mark@codesourcery.com>
// Thread support written by Jason Merrill, Red Hat Inc. <jason@redhat.com>

#include <bits/c++config.h>
#include <cxxabi.h>
#include <exception>
#include <new>
#include <ext/atomicity.h>
#include <ext/concurrence.h>
#include <bits/atomic_lockfree_defines.h>
#if defined(__GTHREADS) && defined(__GTHREAD_HAS_COND) \
  && (ATOMIC_INT_LOCK_FREE > 1) && defined(_GLIBCXX_HAVE_LINUX_FUTEX)
# include <climits>
# include <syscall.h>
# include <unistd.h>
# define _GLIBCXX_USE_FUTEX
# define _GLIBCXX_FUTEX_WAIT 0
# define _GLIBCXX_FUTEX_WAKE 1
#endif

// The IA64/generic ABI uses the first byte of the guard variable.
// The ARM EABI uses the least significant bit.

// Thread-safe static local initialization support.
#ifdef __GTHREADS
# ifndef _GLIBCXX_USE_FUTEX
namespace
{
  // A single mutex controlling all static initializations.
  static __gnu_cxx::__recursive_mutex* static_mutex;  

  typedef char fake_recursive_mutex[sizeof(__gnu_cxx::__recursive_mutex)]
  __attribute__ ((aligned(__alignof__(__gnu_cxx::__recursive_mutex))));
  fake_recursive_mutex fake_mutex;

  static void init()
  { static_mutex =  new (&fake_mutex) __gnu_cxx::__recursive_mutex(); }

  __gnu_cxx::__recursive_mutex&
  get_static_mutex()
  {
    static __gthread_once_t once = __GTHREAD_ONCE_INIT;
    __gthread_once(&once, init);
    return *static_mutex;
  }

  // Simple wrapper for exception safety.
  struct mutex_wrapper
  {
    bool unlock;
    mutex_wrapper() : unlock(true)
    { get_static_mutex().lock(); }

    ~mutex_wrapper()
    {
      if (unlock)
	static_mutex->unlock();
    }
  };
}
# endif

# if defined(__GTHREAD_HAS_COND) && !defined(_GLIBCXX_USE_FUTEX)
namespace
{
  // A single condition variable controlling all static initializations.
  static __gnu_cxx::__cond* static_cond;  

  // using a fake type to avoid initializing a static class.
  typedef char fake_cond_t[sizeof(__gnu_cxx::__cond)]
  __attribute__ ((aligned(__alignof__(__gnu_cxx::__cond))));
  fake_cond_t fake_cond;

  static void init_static_cond()
  { static_cond =  new (&fake_cond) __gnu_cxx::__cond(); }

  __gnu_cxx::__cond&
  get_static_cond()
  {
    static __gthread_once_t once = __GTHREAD_ONCE_INIT;
    __gthread_once(&once, init_static_cond);
    return *static_cond;
  }
}
# endif

# ifndef _GLIBCXX_GUARD_TEST_AND_ACQUIRE

// Test the guard variable with a memory load with
// acquire semantics.

inline bool
__test_and_acquire (__cxxabiv1::__guard *g)
{
  unsigned char __c;
  unsigned char *__p = reinterpret_cast<unsigned char *>(g);
  __atomic_load (__p, &__c,  __ATOMIC_ACQUIRE);
  (void) __p;
  return _GLIBCXX_GUARD_TEST(&__c);
}
#  define _GLIBCXX_GUARD_TEST_AND_ACQUIRE(G) __test_and_acquire (G)
# endif

# ifndef _GLIBCXX_GUARD_SET_AND_RELEASE

// Set the guard variable to 1 with memory order release semantics.

inline void
__set_and_release (__cxxabiv1::__guard *g)
{
  unsigned char *__p = reinterpret_cast<unsigned char *>(g);
  unsigned char val = 1;
  __atomic_store (__p, &val, __ATOMIC_RELEASE);
  (void) __p;
}
#  define _GLIBCXX_GUARD_SET_AND_RELEASE(G) __set_and_release (G)
# endif

#else /* !__GTHREADS */

# undef _GLIBCXX_GUARD_TEST_AND_ACQUIRE
# undef _GLIBCXX_GUARD_SET_AND_RELEASE
# define _GLIBCXX_GUARD_SET_AND_RELEASE(G) _GLIBCXX_GUARD_SET (G)

#endif /* __GTHREADS */

//
// Here are C++ run-time routines for guarded initialization of static
// variables. There are 4 scenarios under which these routines are called:
//
//   1. Threads not supported (__GTHREADS not defined)
//   2. Threads are supported but not enabled at run-time.
//   3. Threads enabled at run-time but __gthreads_* are not fully POSIX.
//   4. Threads enabled at run-time and __gthreads_* support all POSIX threads
//      primitives we need here.
//
// The old code supported scenarios 1-3 but was broken since it used a global
// mutex for all threads and had the mutex locked during the whole duration of
// initialization of a guarded static variable. The following created a
// dead-lock with the old code.
//
//	Thread 1 acquires the global mutex.
//	Thread 1 starts initializing static variable.
//	Thread 1 creates thread 2 during initialization.
//	Thread 2 attempts to acquire mutex to initialize another variable.
//	Thread 2 blocks since thread 1 is locking the mutex.
//	Thread 1 waits for result from thread 2 and also blocks. A deadlock.
//
// The new code here can handle this situation and thus is more robust. However,
// we need to use the POSIX thread condition variable, which is not supported
// in all platforms, notably older versions of Microsoft Windows. The gthr*.h
// headers define a symbol __GTHREAD_HAS_COND for platforms that support POSIX
// like condition variables. For platforms that do not support condition
// variables, we need to fall back to the old code.

// If _GLIBCXX_USE_FUTEX, no global mutex or condition variable is used,
// only atomic operations are used together with futex syscall.
// Valid values of the first integer in guard are:
// 0				  No thread encountered the guarded init
//				  yet or it has been aborted.
// _GLIBCXX_GUARD_BIT		  The guarded static var has been successfully
//				  initialized.
// _GLIBCXX_GUARD_PENDING_BIT	  The guarded static var is being initialized
//				  and no other thread is waiting for its
//				  initialization.
// (_GLIBCXX_GUARD_PENDING_BIT    The guarded static var is being initialized
//  | _GLIBCXX_GUARD_WAITING_BIT) and some other threads are waiting until
//				  it is initialized.

namespace __cxxabiv1 
{
#ifdef _GLIBCXX_USE_FUTEX
  namespace
  {
    static inline int __guard_test_bit (const int __byte, const int __val)
    {
      union { int __i; char __c[sizeof (int)]; } __u = { 0 };
      __u.__c[__byte] = __val;
      return __u.__i;
    }
  }
#endif

  static inline int
  init_in_progress_flag(__guard* g)
  { return ((char *)g)[1]; }

  static inline void
  set_init_in_progress_flag(__guard* g, int v)
  { ((char *)g)[1] = v; }

  static inline void
  throw_recursive_init_exception()
  {
#if __cpp_exceptions
	throw __gnu_cxx::recursive_init_error();
#else
	// Use __builtin_trap so we don't require abort().
	__builtin_trap();
#endif
  }

  // acquire() is a helper function used to acquire guard if thread support is
  // not compiled in or is compiled in but not enabled at run-time.
  static int
  acquire(__guard *g)
  {
    // Quit if the object is already initialized.
    if (_GLIBCXX_GUARD_TEST(g))
      return 0;

    if (init_in_progress_flag(g))
      throw_recursive_init_exception();

    set_init_in_progress_flag(g, 1);
    return 1;
  }

  extern "C"
  int __cxa_guard_acquire (__guard *g) 
  {
#ifdef __GTHREADS
    // If the target can reorder loads, we need to insert a read memory
    // barrier so that accesses to the guarded variable happen after the
    // guard test.
    if (_GLIBCXX_GUARD_TEST_AND_ACQUIRE (g))
      return 0;

# ifdef _GLIBCXX_USE_FUTEX
    // If __atomic_* and futex syscall are supported, don't use any global
    // mutex.

    // Use the same bits in the guard variable whether single-threaded or not,
    // so that __cxa_guard_release and __cxa_guard_abort match the logic here
    // even if __libc_single_threaded becomes false between now and then.

    if (__gnu_cxx::__is_single_threaded())
      {
	// No need to use atomics, and no need to wait for other threads.
	int *gi = (int *) (void *) g;
	if (*gi == 0)
	  {
	    *gi = _GLIBCXX_GUARD_PENDING_BIT;
	    return 1;
	  }
	else
	  throw_recursive_init_exception();
      }
    else
      {
	int *gi = (int *) (void *) g;
	const int guard_bit = _GLIBCXX_GUARD_BIT;
	const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
	const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;

	while (1)
	  {
	    int expected(0);
	    if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
					    __ATOMIC_ACQ_REL,
					    __ATOMIC_ACQUIRE))
	      {
		// This thread should do the initialization.
		return 1;
	      }
	      
	    if (expected == guard_bit)
	      {
		// Already initialized.
		return 0;	
	      }

	     if (expected == pending_bit)
	       {
		 // Use acquire here.
		 int newv = expected | waiting_bit;
		 if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
						  __ATOMIC_ACQ_REL, 
						  __ATOMIC_ACQUIRE))
		   {
		     if (expected == guard_bit)
		       {
			 // Make a thread that failed to set the
			 // waiting bit exit the function earlier,
			 // if it detects that another thread has
			 // successfully finished initialising.
			 return 0;
		       }
		     if (expected == 0)
		       continue;
		   }
		 
		 expected = newv;
	       }

	    syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAIT, expected, 0);
	  }
      }
# else // ! _GLIBCXX_USE_FUTEX
    if (__gthread_active_p ())
      {
	mutex_wrapper mw;

	while (1)	// When this loop is executing, mutex is locked.
	  {
#  ifdef __GTHREAD_HAS_COND
	    // The static is already initialized.
	    if (_GLIBCXX_GUARD_TEST(g))
	      return 0;	// The mutex will be unlocked via wrapper

	    if (init_in_progress_flag(g))
	      {
		// The guarded static is currently being initialized by
		// another thread, so we release mutex and wait for the
		// condition variable. We will lock the mutex again after
		// this.
		get_static_cond().wait_recursive(&get_static_mutex());
	      }
	    else
	      {
		set_init_in_progress_flag(g, 1);
		return 1; // The mutex will be unlocked via wrapper.
	      }
#  else
	    // This provides compatibility with older systems not supporting
	    // POSIX like condition variables.
	    if (acquire(g))
	      {
		mw.unlock = false;
		return 1; // The mutex still locked.
	      }
	    return 0; // The mutex will be unlocked via wrapper.
#  endif
	  }
      }
# endif
#endif // ! __GTHREADS

    return acquire (g);
  }

  extern "C"
  void __cxa_guard_abort (__guard *g) noexcept
  {
#ifdef _GLIBCXX_USE_FUTEX
    // If __atomic_* and futex syscall are supported, don't use any global
    // mutex.

    if (__gnu_cxx::__is_single_threaded())
      {
	// No need to use atomics, and no other threads to wake.
	int *gi = (int *) (void *) g;
	*gi = 0;
	return;
      }
    else
      {
	int *gi = (int *) (void *) g;
	const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
	int old = __atomic_exchange_n (gi, 0, __ATOMIC_ACQ_REL);

	if ((old & waiting_bit) != 0)
	  syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX);
	return;
      }
#elif defined(__GTHREAD_HAS_COND)
    if (__gthread_active_p())
      {	
	mutex_wrapper mw;

	set_init_in_progress_flag(g, 0);

	// If we abort, we still need to wake up all other threads waiting for
	// the condition variable.
        get_static_cond().broadcast();
	return;
      }	
#endif

    set_init_in_progress_flag(g, 0);
#if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND)
    // This provides compatibility with older systems not supporting POSIX like
    // condition variables.
    if (__gthread_active_p ())
      static_mutex->unlock();
#endif
  }

  extern "C"
  void __cxa_guard_release (__guard *g) noexcept
  {
#ifdef _GLIBCXX_USE_FUTEX
    // If __atomic_* and futex syscall are supported, don't use any global
    // mutex.

    if (__gnu_cxx::__is_single_threaded())
      {
	int *gi = (int *) (void *) g;
	*gi = _GLIBCXX_GUARD_BIT;
	return;
      }
    else
      {
	int *gi = (int *) (void *) g;
	const int guard_bit = _GLIBCXX_GUARD_BIT;
	const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
	int old = __atomic_exchange_n (gi, guard_bit, __ATOMIC_ACQ_REL);

	if ((old & waiting_bit) != 0)
	  syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX);
	return;
      }

#elif defined(__GTHREAD_HAS_COND)
    if (__gthread_active_p())
      {
	mutex_wrapper mw;

	set_init_in_progress_flag(g, 0);
	_GLIBCXX_GUARD_SET_AND_RELEASE(g);

        get_static_cond().broadcast();
	return;
      }	
#endif

    set_init_in_progress_flag(g, 0);
    _GLIBCXX_GUARD_SET_AND_RELEASE (g);

#if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND)
    // This provides compatibility with older systems not supporting POSIX like
    // condition variables.
    if (__gthread_active_p())
      static_mutex->unlock();
#endif
  }
}