Re: Whither klists?

[Christoph Hellwig <hch@infradead.org>]

On Thu, Sep 01, 2005 at 02:09:19PM -0400, Alan Stern wrote:
> Patrick and Greg:
> 
> To put it baldly: Should klists be replaced with regular lists, each 
> protected by an rwsem (or even a mutex)?
> 
> The advantage of klists is that threads can remove or add nodes while 
> other threads iterate through the list.  With an rwsem, only one thread 
> would be able to add or remove a node at a time, and only when no other 
> thread was using the list.  Considering that klists are currently used 
> to hold:
> 
> 	the set of all devices on a bus,
> 
> 	the set of all drivers for a bus, and
> 
> 	the set of all children of a device,
> 
> 	(not counting the set of all devices bound to a driver, since
> 	there's already a patch to replace that with a mutex-protected
> 	regular list)
> 
> this limitation on adding or removing doesn't seem significant.  There
> aren't many places where these lists are iterated over or altered.  We
> could remove most of the overhead associated with klists and get rid of an
> extra API for people to learn.
> 
> Note that this would be very different from the old bus subsystem rwsem.  
> That protected too much -- everything associated with the bus subsystem -- 
> making it a pronounced chokepoint.  My suggestion involves a separate 
> rwsem for each of these lists, so that none of them would be subject to 
> much contention.

Might also be worth to do a micro-benchmark for it (maybe in userland).
The current klist code is far too complex for it's own good.