История изменений

 (*) The compiler is within its rights to reorder memory accesses unless
     you tell it not to.  For example, consider the following interaction
     between process-level code and an interrupt handler:

	void process_level(void)
	{
		msg = get_message();
		flag = true;
	}

	void interrupt_handler(void)
	{
		if (flag)
			process_message(msg);
	}

     There is nothing to prevent the compiler from transforming
     process_level() to the following, in fact, this might well be a
     win for single-threaded code:

	void process_level(void)
	{
		flag = true;
		msg = get_message();
	}

     If the interrupt occurs between these two statement, then
     interrupt_handler() might be passed a garbled msg.  Use WRITE_ONCE()
     to prevent this as follows:

	void process_level(void)
	{
		WRITE_ONCE(msg, get_message());
		WRITE_ONCE(flag, true);
	}

	void interrupt_handler(void)
	{
		if (READ_ONCE(flag))
			process_message(READ_ONCE(msg));
	}

 (*) The compiler is within its rights to reorder memory accesses unless
     you tell it not to.  For example, consider the following interaction
     between process-level code and an interrupt handler:

	void process_level(void)
	{
		msg = get_message();
		flag = true;
	}

	void interrupt_handler(void)
	{
		if (flag)
			process_message(msg);
	}

     There is nothing to prevent the compiler from transforming
     process_level() to the following, in fact, this might well be a
     win for single-threaded code:

	void process_level(void)
	{
		flag = true;
		msg = get_message();
	}

     If the interrupt occurs between these two statement, then
     interrupt_handler() might be passed a garbled msg.  Use WRITE_ONCE()
     to prevent this as follows:

	void process_level(void)
	{
		WRITE_ONCE(msg, get_message());
		WRITE_ONCE(flag, true);
	}

	void interrupt_handler(void)
	{
		if (READ_ONCE(flag))
			process_message(READ_ONCE(msg));
	}