Re: Locking in disk(9) api

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Wed, Dec 30, 2009 at 12:32:02AM +0100, Michael van Elst wrote:
> On Tue, Dec 29, 2009 at 01:36:37PM -0800, John Nemeth wrote:
> > } So far, all users have private locks and the disk(9) data is seen
> > } as part of the device instance and is protected by such a private lock.
> > } I'm not convinced that you need a separate lock just for disk(9) data.
> > 
> >      Aren't these the stats displayed by iostat(8) [-dDx]?  Do we need
> > locking shared with the things that read the stats?  Or, is it
> > considered to be okay if the reader gets stats that are in flux?
> 
> Well, originally this was read via kvm and always in a state of flux.

I think this is almost the canonical example of data we should not lock.
The cost of using locking *or* atomic operations for statistics update
will be extreme on busy MP systems, and there's almost no real benefit.