Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v10)

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 20 Apr 2009 16:44:46 -0700

On Tue, Apr 21, 2009 at 08:41:36AM +1000, Paul Mackerras wrote:
> Paul E. McKenney writes:
> 
> > But a single CPU is acquiring one lock per CPU, so all the increments
> > are to one CPU's preempt_count.  :-(
> 
> OK, I see, so a task can't take more than 255 spinlocks without
> overflowing the preempt count, which seems a bit limiting.
> 
> There are 6 free bits in the preempt_count currently, so the preempt
> count could be expanded to 14 bits, which would be enough for all
> current systems.  Beyond that I guess we could make preempt_count be a
> long and allow bigger counts on 64-bit architectures.

Or we use the trick Eric suggested and Steve employed in the most recent
patch.  ;-)

An alternative would be for the update code to acquire but one lock at a
time, but this would likely require another lock to exclude other
updaters and I believe would also require restructuring the count
accumulation.

So Steve's current patch seems a bit less intrusive, overall.

							Thanx, Paul