linux-kernel - Re: memory barrier question

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Message-ID: <20100918021219.1ff6f98f@lxorguk.ukuu.org.uk>
Date:	Sat, 18 Sep 2010 02:12:19 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	paulmck@...ux.vnet.ibm.com, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: memory barrier question

> >    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.

Actually I've met several compilers which will optimise them but not gcc
and not in a Linux environment. Some compilers know how to optimise the
== NULL case for pointer dereferences providing their programming
environment has a couple of low pages mapped read only and reading 0.

Alan
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