Re: memory barrier question

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@szeredi.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.

> Funny that such a bug could stay unnoticed in so often excercised
> code.  Yeah I know it's alpha only.  I wonder how much of this pattern
> exists in the kernel elswhere without the necessary barriers.

The probability of failure is low, but non-zero.  :-/

							Thanx, Paul