From: Keith Owens <kaos@ocs.com.au>
To: Alan Stern <stern@rowland.harvard.edu>
cc: Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Notifier chains are unsafe 
In-reply-to: Your message of "Mon, 24 Oct 2005 16:48:58 -0400."
             <Pine.LNX.4.44L0.0510241634410.4448-100000@iolanthe.rowland.org> 
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Date: Wed, 26 Oct 2005 16:11:46 +1000
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On Mon, 24 Oct 2005 16:48:58 -0400 (EDT), 
Alan Stern <stern@rowland.harvard.edu> wrote:
>Has anyone been bothered by the fact that notifier chains are not safe 
>with regard to registration and unregistration while the chain is in use?
>The notifier_chain_register and notifier_chain_unregister routines have 
>writelock protections, but the corresponding readlock is never taken!
>
>It shouldn't be hard to make this work safely, even allowing such things
>as notifier routines unregistering themselves as they run.  The patch
>below contains an example implementation, showing one way to do it.
>
>But doing this correctly requires knowing how notifier chains are used.  
>
>	Are they always called in process context, with interrupts enabled?
>
>	Or do some get called in interrupt context?

Register and unregister should only be called from contexts that can
sleep, although that may not be documented.  notifier_call_chain() can
be called in any context, it must not take any locks.

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