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:

> On Thu, Dec 05, 2013 at 10:05:47AM -0800, Paul E. McKenney wrote:
> > On Thu, Dec 05, 2013 at 10:50:42AM +0100, Ingo Molnar wrote:
> > > 
> > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > 
> > > > + (*) The compiler is within its rights to reload a variable, for example,
> > > > +     in cases where high register pressure prevents the compiler from
> > > > +     keeping all data of interest in registers.  The compiler might
> > > > +     therefore optimize the variable tmp out of our previous example:
> > > > +
> > > > +	while (tmp = a)
> > > > +		do_something_with(tmp);
> > > > +
> > > > +     This could result in the following code, which is perfectly safe in
> > > > +     single-threaded code, but can be fatal in concurrent code:
> > > > +
> > > > +	while (a)
> > > > +		do_something_with(a);
> > > > +
> > > > +     For example, the optimized version of this code could result in
> > > > +     passing a zero to do_something_with() in the case where the variable
> > > > +     a was modified by some other CPU between the "while" statement and
> > > > +     the call to do_something_with().
> > > 
> > > Nit: I guess references to variable names such as 'a' should be quoted 
> > > (same for 'tmp', 'b', etc):
> > > 
> > >         For example, the optimized version of this code could result in
> > >         passing a zero to do_something_with() in the case where the variable
> > >         'a' was modified by some other CPU between the "while" statement and
> > >         the call to do_something_with().
> > > 
> > > which makes reading it less ambiguous and more fluid IMO. This 
> > > observation applies to the whole document as 'a' is used in many 
> > > places.
> > 
> > Good point, fixed.
> 
> Which reminds me -- the thing that makes me most nervous about 
> prohibiting speculative stores is the bit about having to anticipate 
> all compiler optimizations that might get rid of the needed 
> conditionals.
> 
> Thoughts?

As long as current compiler versions behave I don't the potential of 
future problems is a problem that can (or should) be solved via 
documentation - there will always be a colorful tension between 
specification and reality, both at the hardware, the code and the 
compiler level ;-)

It doesn't hurt to outline our expectations in any case, agreed?

Thanks,

	Ingo