Re: [rfc] "fair" rw spinlocks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Nov 30, 2009 at 08:07:16AM -0800, Linus Torvalds wrote:
> 
> 
> On Mon, 30 Nov 2009, Nick Piggin wrote:
> > 
> > Well the simple thing I tried earlier was a per-cpu array of nesting
> > counter there. It's not _too_ expensive, but it does add another cacheline
> > access and branch there. It seems to work in solving the livelock though.
> 
> So how did you do the nesting counter? Afaik, it needs to be something 
> like
> 
> 	local_irq_save(flags);
> 	if (!get_cpu_var(tasklist_counter)++)
> 		spin_lock(&tasklist_lock);
> 	local_irq_restore(flags);
> 
> on the read_lock side (and the same in reverse on unlock). Which seems 
> quite a bit more expensive than what we have now. Especially on UP, but I 
> guess you can make it conditional on CONFIG_SMP (but that won't help 
> generic kernels).

My suggestion would be to put the nesting counter in the task structure
to avoid this problem.

							Thanx, Paul