[patch] mm: fix lockless pagecache reordering bug (was Re: BUG: soft lockup - is this XFS problem?)

[Paul E. McKenney]

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.

Whatever the answer, I would argue for -at- -least- a comment explaining
why it is safe.  I am not seeing the objection to rcu_dereference(), but
I must confess that it has been awhile since I have looked closely at
the radix_tree code.  :-/

							Thanx, Paul