Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

Alan Stern
2009-12-08T19:30:44Z

On Tue, 8 Dec 2009, Linus Torvalds wrote:

> > The whole readers vs. writers thing is a non-sequitur.
>
> No it's not.
>
> It's a 100% equivalent problem. It's purely a change of wording. The end
> result is the same.

Well, of course the end result is the same (ignoring bugs) -- that was
the point. It doesn't follow that the two locking mechanisms are 100%
equivalent.

> And note how even though you sprinkled random memory barriers around, you
> still got it wrong.

Yes. That comes of trying to think at the keyboard.

> It's certainly not smaller. It's not faster. It doesn't have support for
> lockdep. And it's BUGGY.

Lockdep will choke on the rwsem approach anyway. It has never been
very good at handling tree-structured locking, especially when there
are non-parent-child interactions. But never mind.

> Really. Tell me why you want to re-implement an existing lock - badly.

I didn't want to. The whole exercise was intended to make a point --
that rwsems do more than we really need here.

> [ Hint: you need a smp_mb() *before* the atomic_dec() in wait-unlock, so
> that anybody else who sees the new value will be guaranteed to have seen
> anything else the unlocker did.

Yes.

> You also need a smp_mb() in the wait_for_lock(), not a smp_rmb(). Can't
> allow writes to migrate up either. 'atomic_read()' does not imply any
> barriers.

No, that's not needed. Unlike reads, writes can't move in front of
data or control dependencies. Or so I've been lead to believe...

> That "wait_for_lock()" is equivalent to a 'read_lock()+read_unlock()'.

Not really. It also corresponds to a 'write_lock()+write_unlock()' (in
the suspend routine). Are you claiming these two compound operations
are equivalent?

> We
> _could_ expose such a mechanism for rwsem's too, but why do it? It's
> actually nicer to use a real read-lock - and do it _around_ the operation,
> because now the locking also automatically gets things like overlapping
> suspends and resumes right.
>
> (Which you'd obviously hope never happens, but it's nice from a conceptual
> standpoint to know that the locking is robust).

> Take heed. You got it wrong. Admit it. Locking is _hard_. SMP memory
> ordering is HARD.

Oh, there's no question about that. I never seriously intended this
stuff to be adopted. It was just for discussion.

Alan Stern