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Date: Sat, 9 Feb 2008 10:01:33 -0800
From: Arjan van de Ven <arjan@infradead.org>
To: Pekka Paalanen <pq@iki.fi>
Cc: Ingo Molnar <mingo@elte.hu>, Christoph Hellwig <hch@infradead.org>,
       linux-kernel@vger.kernel.org, pq@iki.fi
Subject: Re: [RFC PATCH] x86: explicit call to mmiotrace in do_page_fault()
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>  	default n
>  	help
>  	  This will build a kernel module called mmiotrace.
> +	  Making this a built-in is heavily discouraged.

why is this? Wouldn't it be nice if distros just shipped with this in their kernel by default
for example?

> -#ifdef CONFIG_PAGE_FAULT_HANDLERS
> -static HLIST_HEAD(pf_handlers); /* protected by RCU */
> -static DEFINE_SPINLOCK(pf_handlers_writer);
> +#ifdef CONFIG_MMIOTRACE_HOOKS
> +static pf_handler_func mmiotrace_pf_handler; /* protected by RCU */
> +static DEFINE_SPINLOCK(mmiotrace_handler_lock);
>  
> -void register_page_fault_handler(struct pf_handler *new_pfh)
> +int mmiotrace_register_pf(pf_handler_func new_pfh)
>  {
> +	int ret = 0;
>  	unsigned long flags;
> -	spin_lock_irqsave(&pf_handlers_writer, flags);
> -	hlist_add_head_rcu(&new_pfh->hlist, &pf_handlers);
> -	spin_unlock_irqrestore(&pf_handlers_writer, flags);
> +	spin_lock_irqsave(&mmiotrace_handler_lock, flags);
> +	if (mmiotrace_pf_handler)
> +		ret = -EBUSY;
> +	else
> +		mmiotrace_pf_handler = new_pfh;
> +	spin_unlock_irqrestore(&mmiotrace_handler_lock, flags);
> +	return ret;
>  }
> -EXPORT_SYMBOL_GPL(register_page_fault_handler);
> +EXPORT_SYMBOL_GPL(mmiotrace_register_pf);

why not use a weak function instead? should be a ton simpler.