Re: + kernel-locking-lockdepc-make-lockdep-initialize-itself-on-demand.patch added to -mm tree

[Ingo Molnar]

* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> > and it should happen in a well defined place, not be opportunistic (and 
> > relatively random) like this, making it dependent on config options and 
> > calling contexts.
> 
> That's an unusable assertion, sorry.
> 
> Initializing lockdep in the above manner guarantees that it is initialized 
> before it is used.  It is *much* more reliable than "try to initialize it before 
> some piece of code which hasn't even been written yet tries to take a lock".

So I didn't like that patch because it called into lockdep in a messy way, without 
having any real knowledge about whether it's safe to do. Should lockdep ever grow 
more complex initialization, such a solution could break in subtle ways. I prefer 
clearly broken code with static dependencies over context-dependent broken code 
with dynamic call ordering/dependencies.

Fortunately we don't have to apply the patch:

> The conceptual problem is that if some piece of code does spin_lock_init() or 
> DEFINE_SPINLOCK(), that lock isn't necessarily initialized yet.

The conceptual problem is that the data structures are not build time initialized 
- but the hlist conversion patch solves that problem nicely!

So I'm a happy camper.

Thanks,

	Ingo