Re: [PATCH tip/core/locking 4/4] Documentation/memory-barriers.txt: Document ACCESS_ONCE()

[Jonathan Corbet <corbet@lwn.net>]

On Wed,  4 Dec 2013 14:46:59 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> The situations in which ACCESS_ONCE() is required are not well documented,
> so this commit adds some verbiage to memory-barriers.txt.

[...]

> +     But please note that the compiler is also closely watching what you
> +     do with the value after the ACCESS_ONCE().  For example, suppose you
> +     do the following and MAX is a preprocessor macro with the value 1:
> +
> +	for ((tmp = ACCESS_ONCE(a)) % MAX)
> +		do_something_with(tmp);

That sure looks like it was meant to be "while" instead of "for"?

[...]

> + (*) 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 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 ACCESS_ONCE()
> +     to prevent this as follows:
> +
> +	void process_level(void)
> +	{
> +		ACCESS_ONCE(msg) = get_message();
> +		ACCESS_ONCE(flag) = true;
> +	}
> +
> +	void interrupt_handler(void)
> +	{
> +		if (ACCESS_ONCE(flag))
> +			process_message(ACCESS_ONCE(msg));
> +	}

Looking at this, I find myself wondering why you couldn't just put a
barrier() between the two statements in process_level()?  ACCESS_ONCE()
seems like a heavy hammer to just avoid reordering of two assignments.
What am I missing, and what could be added here to keep the other folks as
dense as me from missing the same thing?

Thanks,

jon