linux-kernel - Re: memory barrier question

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Message-Id: <E1OwHkg-0007IX-DH@pomaz-ex.szeredi.hu>
Date:	Thu, 16 Sep 2010 18:56:02 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	paulmck@...ux.vnet.ibm.com
CC:	miklos@...redi.hu, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: memory barrier question

On Thu, 16 Sep 2010, Paul E. McKenney wrote:
> On Thu, Sep 16, 2010 at 06:06:53PM +0200, Miklos Szeredi wrote:
> > On Thu, 16 Sep 2010, Paul E. McKenney wrote:
> > > On Thu, Sep 16, 2010 at 03:30:56PM +0100, David Howells wrote:
> > > > 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?
> > > 
> > > Are the path_get()s protecting this?
> > 
> > No, when creating a file the dentry will go from negative to positive
> > independently from lookup.  The dentry can get instantiated with an
> > inode between the path_get() and dereferencing ->d_inode.
> > 
> > > If there is no protection, then something like rcu_dereference() is
> > > needed for the assignment from next.dentry->d_inode.
> > 
> > Do I understand correctly that the problem is that a CPU may have a
> > stale cache associated with *inode, one that was loaded before the
> > write barrier took effect?
> 
> Yes, especially if the compiler is aggressively optimizing.

How do compiler optimizations make a difference?

Thanks,
Miklos
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