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

[Peter Zijlstra]

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.