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

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Date:	Mon, 20 Apr 2009 16:44:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Evgeniy Polyakov <zbr@...emap.net>,
	David Miller <davem@...emloft.net>, kaber@...sh.net,
	torvalds@...ux-foundation.org, jeff.chua.linux@...il.com,
	mingo@...e.hu, laijs@...fujitsu.com, jengelh@...ozas.de,
	r000n@...0n.net, linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	benh@...nel.crashing.org, mathieu.desnoyers@...ymtl.ca
Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v10)

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
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