From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v10)
Date: Tue, 21 Apr 2009 08:41:36 +1000
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Cc: Eric Dumazet <dada1@cosmosbay.com>,
	Stephen Hemminger <shemminger@vyatta.com>,
	Evgeniy Polyakov <zbr@ioremap.net>,
	David Miller <davem@davemloft.net>, kaber@trash.net,
	torvalds@linux-foundation.org, jeff.chua.linux@gmail.com,
	mingo@elte.hu, laijs@cn.fujitsu.com, jengelh@medozas.de,
	r000n@r000n.net, linux-kernel@vger.kernel.org,
	netfilter-devel@vger.kernel.org, netdev@vger.kernel.org,
	benh@kernel.crashing.org, mathieu.desnoyers@polymtl.ca
To: paulmck@linux.vnet.ibm.com
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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.