linux-kernel - Re: [PATCH] netfilter: use per-cpu recursive lock (v13)

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Message-ID: <alpine.LFD.2.00.0904220821240.3101@localhost.localdomain>
Date:	Wed, 22 Apr 2009 08:32:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Stephen Hemminger <shemminger@...tta.com>
cc:	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	paulmck@...ux.vnet.ibm.com, Eric Dumazet <dada1@...mosbay.com>,
	Evgeniy Polyakov <zbr@...emap.net>,
	David Miller <davem@...emloft.net>, kaber@...sh.net,
	jeff.chua.linux@...il.com, 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 (v13)

On Tue, 21 Apr 2009, Stephen Hemminger wrote:
>
> This version of x_tables (ip/ip6/arp) locking uses a per-cpu
> recursive lock that can be nested.

Ack on the code.

But the comment is _still_ crap. Please update. It's not a recursive lock, 
as clearly shown by the code itself. It's a per-cpu read-write lock, and 
only the reader is "recursive" (but that's how read-write locks with in 
Linux, and that has nothing to do with anything).

So make the explanations match the code and the intent. Write it something 
like

	This version of x_tables (ip/ip6/arp) locking uses a per-cpu
	reader-writer lock lock where the readers can nest.

and don't confuse it with incorrect commit messages. The lock is very much 
not recursive - on purpose - for half the people taking it.

[ That, btw, was always true, even in the original random open-coded 
  version. Because you can't actually do a real recursive lock without 
  having notion of "current ownership" either by making the count be 
  <per-thread,per-lock> - like the BKL - or by saving the ownership 
  information in the lock. A plain counter simply doesn't do it. ]

		Linus
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