Re: memory barrier question

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Sep 18, 2010 at 07:49:08AM +1000, Benjamin Herrenschmidt wrote:
> 
> > Right but in the concrete namei example I can't see how a compiler
> > optimization can make a difference.  The order of the loads is quite
> > clear:
> > 
> >    LOAD inode = next.dentry->inode
> >    if (inode != NULL)
> >    	LOAD inode->f_op
> > 
> > What is there the compiler can optimize?
> 
> Those two loads depend on each other, I don't think any implementation
> can re-order them. In fact, such data dependency is typically what is
> used to avoid having barriers in some cases. The second load cannot be
> issued until the value from the first one is returned.

Sufficiently sadistic compiler and CPU implementations could do value
speculation, for example, driven by profile-feedback optimization.
Then the guess might initially incorrect, but then a store by some other
CPU could make the subsequent test decide (wrongly) that the guess had
in fact been correct.

Needless to say, I am not a fan of value speculation.  But other people
do like it a lot.

							Thanx, Paul