Re: locking order between softnet_lock and the kernel lock

[David Laight <david%l8s.co.uk@localhost>]

On Mon, Mar 26, 2012 at 10:09:17PM -0400, Mouse wrote:
> > Generally speaking, I don't think we require locks to be released in
> > order - though of course it is good practice.
> 
> If you're going to release them all, then, unless the locks themselves
> interact somehow (as someone said was the case with kernel-lock, I
> think), then you can release in whatever order you please - _provided_
> that you release them all without reacquiring any until you've finished
> releasing.
> 
> I do, however, agree that proper nesting is usually a good idea.  Just
> not for correctness reasons.

There are some very good reasons to allow locks be released in any order.
Consider some list/hash/tree of items, lookup sequence:
1) take global lock
2) find item
3) lock item
4) release global lock

Otherwise you have to perform a dance with a reference count.
(Or keep everything locked for much longer.)

Clearly you can't reacquire the global lock (eg to remove an item)
but that isn't necessarily a problem as items can just be marked
'deleted' and pruned from the global structure at a later date
(eg on insert).

There are issues with correct restoring of SPL levels. I think netbsd
stores the saved spl in the kernel thread (not the mutex) and restores
it when the last mutex is released - this isn't completely optimal!

        David

-- 
David Laight: david%l8s.co.uk@localhost