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Subject: Re: [PATCH] (4/5) improved notifier callback mechanism - read copy
	update
From: Stephen Hemminger <shemminger@osdl.org>
To: vamsi@in.ibm.com, John Levon <levon@movementarian.org>
Cc: Linus Torvalds <torvalds@transmeta.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Kernel List <linux-kernel@vger.kernel.org>
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	 <20021219181929.A5265@in.ibm.com>
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Thanks, will look into it in more detail. Perhaps figuring out how to do
oprofile without sleeping would be best.

On Thu, 2002-12-19 at 04:49, Vamsi Krishna S . wrote:
> Hi Stephen,
> 
> On Wed, Dec 18, 2002 at 11:06:08PM +0000, Stephen Hemminger wrote:
> > The notifier interface was only partially locked. The
> > notifier_call_chain needs to be called in places where it is impossible
> > to safely without having deadlocks; for example, NMI watchdog timeout.
> > 
> > This patch uses read-copy-update to manage the list.  One extra bit of
> > safety is using a reference count on the notifier_blocks to allow for
> > cases like oprofile which need to sleep in a callback.
> > 
> <snip>
> >   
> >  int notifier_call_chain(struct list_head *list, unsigned long val, void
> > *v)
> >  {
> > -	struct list_head *p;
> > +	struct list_head *p, *nxtp;
> >  	int ret = NOTIFY_DONE;
> >  
> > -	list_for_each(p, list) {
> > +	rcu_read_lock();
> > +	list_for_each_safe_rcu(p, nxtp, list) {
> >  		struct notifier_block *nb =
> >  			list_entry(p, struct notifier_block, link);
> >  
> > +		atomic_inc(&nb->inuse);
> >  		ret = nb->notifier_call(nb,val,v);
> > +		atomic_dec(&nb->inuse);
> > +
> 
> There could be a small problem here. When rcu_read_lock() is called,
> it bumps the preempt_count, so when the called handler attempts
> to sleep, it will oops with "Bad: scheduling in atomic region".
-- 
Stephen Hemminger <shemminger@osdl.org>
Open Source Devlopment Lab

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