Re: [patch] mm: fix lockless pagecache reordering bug (was Re:

Paul E. McKenney
2009-01-05T21:58:43Z

On Mon, Jan 05, 2009 at 10:04:51PM +0100, Peter Zijlstra wrote:
> On Mon, 2009-01-05 at 12:12 -0800, Paul E. McKenney wrote:
> > On Mon, Jan 05, 2009 at 10:44:27AM -0800, Linus Torvalds wrote:
> > > On Mon, 5 Jan 2009, Nick Piggin wrote:
> > > > On Mon, Jan 05, 2009 at 09:30:55AM -0800, Linus Torvalds wrote:
> > > > Putting an rcu_dereference there might work, but I think it misses a
> > > > subtlety of this code.
> > >
> > > No, _you_ miss the subtlety of something that can change under you.
> > >
> > > Look at radix_tree_deref_slot(), and realize that without the
> > > rcu_dereference(), the compiler would actually be allowed to think that it
> > > can re-load anything from *pslot several times. So without my one-liner
> > > patch, the compiler can actually do this:
> > >
> > > register = load_from_memory(pslot)
> > > if (radix_tree_is_indirect_ptr(register))
> > > goto fail:
> > > return load_from_memory(pslot);
> > >
> > > fail:
> > > return RADIX_TREE_RETRY;
> >
> > My guess is that Nick believes that the value in *pslot cannot change
> > in such as way as to cause radix_tree_is_indirect_ptr()'s return value
> > to change within a given RCU grace period, and that Linus disagrees.
>
> Nick's belief would indeed be true IFF all modifying ops including all
> uses of radix_tree_replace_slot() are serialized wrt. each other.
>
> However, since radix_tree_deref_slot() is the counterpart of
> radix_tree_replace_slot(), one would indeed expect rcu_dereference()
> therein, much like Linus suggests.
>
> While what Nick says is true, the lifetime management of the data
> objects is arranged externally from the radix tree -- I still think we
> need the rcu_dereference() even for that argument, as we want to support
> RCU lifetime management as well.

Makes sense to me!

Thanx, Paul