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

[Andrew Morton]

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