Re: Notifier chains are unsafe

[Chandra Seetharaman <sekharan@us.ibm.com>]

On Wed, 2005-10-26 at 23:44 +0200, Andi Kleen wrote:
> On Wednesday 26 October 2005 22:40, Alan Stern wrote:
> l> On Wed, 26 Oct 2005, Andreas Kleen wrote:
> > 
> > > > Note that the RCU documentation says RCU critical sections are not
> > > > allowed
> > > > to sleep.
> > > 
> > > In this case it would be ok.
> > 
> > I don't understand.  If it's okay for an RCU critical section to sleep in 
> > this case, why wouldn't it be okay always?  What's special here?
> > 
> > Aren't there requirements about critical sections finishing on the same 
> > CPU as they started on?
> 
> 
> Like I wrote earlier: as long as the notifier doesn't unregister itself
> the critical RCU section for the list walk is only a small part of notifier_call_chain.
> It's basically a stable anchor in the list that won't change.

Andy, comment above rcu_read_lock says, "It is illegal to block while in
an RCU read-side critical section."

As i mentioned in the other email we are discussing about "task
notifier" in lse-tech. We thought of using RCU, but one of the
requirements was that the registered function should be able to block,
so we are looking for alternatives.

> 
> The only change needed would be to make these parts unpreemptable and of course
> add a RCU step during unregistration.
> 
> -Andi
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-- 

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    Chandra Seetharaman               | Be careful what you choose....
              - sekharan@us.ibm.com   |      .......you may get it.
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