Re: Notifier chains are unsafe

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

On Fri, 2005-10-28 at 10:23 -0400, Alan Stern wrote:
> On Thu, 27 Oct 2005, Chandra Seetharaman wrote:
> 
> > So, requirements to fix the bug are:
> > 	- no sleeping in register/unregister(if we want to keep the
> >           current way of use. We can change it and make the relevant
> >           changes in the kernel code, if it is agreeable)
> 
> I think we will have to make these changes.  In principal it shouldn't be 
> hard to add a simple "enabled" flag to each callout which currently is
> registered/unregistered atomically or while running.  We could even put 
> such a flag into the notifier_block structure and add routines to set or 
> clear it, using appropriate barriers.

I do not understand the purpose of enabled flag. Can you clarify
> 
> > 	- notifier_call_chain could be called from any context
> > 	- callout function could sleep
> > 	- no acquiring locks in notifier_call_chain
> >         - make sure the list is consistent :) (which is problem Alan
> >           started to fix)
> > 	- anything else ?
> 
> Let's clarify the "list is consistent" statement.  Obviously it implies 
> that no more than one thread can modify the list pointers at any time.  
> Beyond that, there should be a guarantee that when unregister returns, the 
> routine being removed is not in use and will not be called by any thread.  
> Likewise, after register returns, any invocation of notifier_call_chain 
> should see the new routine.

true
> 
> Alan Stern
> 
> 
-- 

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