Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores

[Peter Zijlstra]

On Wed, Feb 24, 2016 at 01:40:13PM -0800, Paul E. McKenney wrote:
> > > +  (*) If both legs of the "if" statement begin with identical stores to
> > > +      the same variable, then those stores must be ordered, either by
> > > +      preceding both of them with smp_mb() or by using smp_store_release()
> > > +      to carry out the stores.  Please note that it is -not- sufficient
> > > +      to use barrier() at beginning of each leg of the "if" statement,
> > > +      as optimizing compilers do not necessarily respect barrier()
> > > +      in this case.

> Let's take the example, replace barrier() with smp_mb(), and see what
> happens:
> 
> 	q = READ_ONCE(a);
> 	if (q) {
> 		smp_mb();
> 		WRITE_ONCE(b, p);
> 		do_something();
> 	} else {
> 		smp_mb();
> 		WRITE_ONCE(b, p);
> 		do_something_else();
> 	}

Why would an optimizing compiler be allowed to lift _anything_ over a
barrier() ? Isn't that a bug?

I thought the whole point of barrier() was to tell the compiler to not
do such things.