linux-kernel - Re: memory barrier question

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Date:	Thu, 16 Sep 2010 15:30:56 +0100
From:	David Howells <dhowells@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dhowells@...hat.com, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: memory barrier question

Miklos Szeredi <miklos@...redi.hu> wrote:

> Is the rmb() really needed?
> 
> Take this code from fs/namei.c for example:
> 
> 		inode = next.dentry->d_inode;
> 		if (!inode)
> 			goto out_dput;
> 
> 		if (inode->i_op->follow_link) {
> 
> It happily dereferences dentry->d_inode without a barrier after
> checking it for non-null, while that d_inode might have just been
> initialized on another CPU with a freshly created inode.  There's
> absolutely no synchornization with that on this side.

Perhaps it's not necessary; once set, how likely is i_op to be changed once
I_NEW is cleared?

> Isn't the fact that we check the pointer for being non-null (together
> with locking/barrier on the other side) enough to ensure that it's
> safe to dereference it?

It's possible that since there's a dependency between the variables on the
reading CPU that the barrier is not required.

I think I have to refer that question to Paul.

David
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