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

[Ingo Molnar <mingo@kernel.org>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> > [...]
> > 
> > > + (*) 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?
> 
> You could use barrier() from an ordering viewpoint.  However, 
> ACCESS_ONCE() is often lighter weight than barrier().  ACCESS_ONCE() 
> affects only that one access, while barrier() forces the compiler to 
> forget pretty much anything it previously gleaned from any region of 
> memory, including private locations that no one else touches.
> 
> I am adding a sentence saying that pure ordering can be provided by 
> barrier(), though often at higher cost.

I suspect a related question would be, is the compiler allowed to 
reorder:


	x = ACCESS_ONCE(a);
	y = ACCESS_ONCE(b);

?

This wording:

+ [...] Howevever, ACCESS_ONCE() can be thought of as a weak form
+for barrier() that affects only the specific accesses flagged by the
+ACCESS_ONCE().

Does not seem to be obvious enough to me - does it affect accesses to 
the variables referenced (but still allows accesses to separate 
variables reordered), or does it affect compiler-ordering of all 
ACCESS_ONCE() instances, instructing the compiler to preserve program 
order?

Also, it's not clear what happens if non-ACCESS_ONCE() access to a 
variable is mixed with ACCESS_ONCE() access.

Thanks,

	Ingo