linux-kernel - Re: memory barrier question

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6383.1284647456@redhat.com>
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/