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Subject: Re: [RFC PATCH] x86: explicit call to mmiotrace in do_page_fault()
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Pekka Paalanen <pq@iki.fi>, Ingo Molnar <mingo@elte.hu>,
       Christoph Hellwig <hch@infradead.org>, linux-kernel@vger.kernel.org
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Date: Sat, 09 Feb 2008 19:39:47 +0100
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On Sat, 2008-02-09 at 10:01 -0800, Arjan van de Ven wrote:
> >  	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?

so default m?

> > -#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.

AFAIC weak functions are a built time thing, not fixed up on module
load. So I'm not seeing the applicability there.

Also, it would force a function call, even for the unlikely case the
tracer is set.