linux-kernel - Re: memory barrier question

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Date:	Sun, 19 Sep 2010 12:47:01 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: memory barrier question

On Fri, 2010-09-17 at 16:12 -0700, Paul E. McKenney wrote:
> 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.

Well, this verges on insanity... we get to a point where nobody's going
to get any code right :-)

I don't think the powerpc arch allows that, that leaves us with the
compiler, but so far I don't think gcc is -that- crazy. Those constructs
are common enough...

Cheers,
Ben.


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