On Thu, Sep 16, 2010 at 03:30:56PM +0100, David Howells wrote:
> Miklos Szeredi <miklos@xxxxxxxxxx> 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?
If there is no protection, then something like rcu_dereference() is
needed for the assignment from next.dentry->d_inode.
> > 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.
We would need either one of the rcu_dereference() or smp_read_barrier_depends()
APIs to enforce the dependency, for example, against the compiler.
Thanx, Paul
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