linux-kernel - Re: memory barrier question

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Message-ID: <20100917231222.GA3060@linux.vnet.ibm.com>
Date:	Fri, 17 Sep 2010 16:12:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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 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
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