linux-kernel - Re: [PATCH 1/2] lockdep missing barrier()

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Date:	Tue, 23 Jan 2007 20:26:37 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, ltt-dev@...fik.org,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	systemtap@...rces.redhat.com, Thomas Gleixner <tglx@...utronix.de>,
	Richard J Moore <richardj_moore@...ibm.com>
Subject: Re: [PATCH 1/2] lockdep missing barrier()

On Tue, 16 Jan 2007 12:56:24 -0500
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> This patch adds a barrier() to lockdep.c lockdep_recursion updates. This
> variable behaves like the preemption count and should therefore use similar
> memory barriers.
> 
> This patch applies on 2.6.20-rc4-git3.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> 
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -166,12 +166,14 @@ static struct list_head chainhash_table[CHAINHASH_SIZE];
>  void lockdep_off(void)
>  {
>  	current->lockdep_recursion++;
> +	barrier();
>  }
>  
>  EXPORT_SYMBOL(lockdep_off);
>  
>  void lockdep_on(void)
>  {
> +	barrier();
>  	current->lockdep_recursion--;
>  }

I am allergic to undocumented barriers.  It is often unobvious what the
barrier is supposed to protect against, yielding mystifying code.  This is
one such case.

Please add code comments.
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