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

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Message-ID: <18924.64032.103954.171918@cargo.ozlabs.ibm.com>
Date:	Tue, 21 Apr 2009 08:41:36 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	paulmck@...ux.vnet.ibm.com
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)

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.

Paul.
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