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

Linus Torvalds
2009-01-05T20:40:26Z

On Mon, 5 Jan 2009, Paul E. McKenney wrote:
>
> 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.

Oh, it's entirely possible that there are some lifetime rules or others
that make it impossible for things to go from "not indirect" ->
"indirect". So if that was Nick's point, then I'm not "disagreeing" per
se.

What I'm disagreeing about is that Nick apparently thinks that this is all
subtle code, and as a result we should add barriers in some very
non-obvious places.

While _I_ think that the problem isn't properly solved by barriers, but by
just making the code less subtle. If the barrier only exists because of
the reload issue, then the obvious solution - to me - is to just use what
is already the proper accessor function that forces a nice reload. That
way the compiler is forced to create code that does what the source
clearly means it to do, regardless of any barriers at all.

Barriers in general should be the _last_ thing added. And if they are
added, they should be added as deeply in the call-chain as possible, so
that we don't need to add them in multiple call-sites. Again, using the
rcu_dereference() approach seems to solve that issue too - rather than add
three barriers in three different places, we just add the proper
dereference in _one_ place.

> 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. :-/

And I'm actually suprised that gcc can generate the problematic code in
the first place. I'd expect that a "atomic_add_unless()" would always be
at LEAST a compiler barrier, even if it isn't necessarily a CPU memory
barrier.

But because we inline it, and because we allow gcc to see that it doesn't
do anything if it gets just the right value from memory, I guess gcc ends
up able to change the "for()" loop so that the first iteration can exit
specially, and then for that case (and no other case) it can cache
variables over the whole atomic_add_unless().

Again, that's very fragile. The fact that Documentation/atomic_ops.txt
says that the failure case doesn't contain any barriers is really _meant_
to be about the architecture-specific CPU barriers, not so much about
something as simple as a compiler re-ordering.

So while I think that we should use rcu_dereference() (regardless of any
other issues), I _also_ think that part of the problem really is the
excessive subtlety in the whole code, and the (obviously very surprising)
fact that gcc could end up caching an unrelated memory load across that
whole atomic op.

Maybe we should make atomics always imply a compiler barrier, even when
they do not imply a memory barrier. The one exception would be the
(special) case of "atomic_read()/atomic_set()", which don't really do any
kind of complex operation at all, and where we really do want the compiler
to be able to coalesce multiple atomic_reads() to a single one.

In contrast, there's no sense in allowing the compiler to coalesce a
"atomic_add_unless()" with anything else. Making it a compiler barrier
(possibly by uninlining it, or just adding a barrier to it) would also
have avoided the whole subtle case - which is always a good thing.

Linus