Re: [PATCH] Avoid calling down_read and down_write during startup

[Benjamin LaHaise <bcrl@kvack.org>]

On Fri, Feb 24, 2006 at 11:44:23AM -0500, Alan Stern wrote:
> In that case you should be worried not about acquiring and releasing the 
> rwsem at the beginning and end of blocking_notifier_call_chain; you should 
> be worried about all the RCU serialization in the core 
> notifier_call_chain routine.

RCU doesn't synchronize readers.

> The atomic chains are a different matter.  The ones that don't run in NMI 
> context could use an rw-spinlock for protection, allowing them also to 
> avoid memory barriers while going through the list.  The notifier chains 
> that do run in NMI don't have this luxury.  Fortunately I don't think 
> there are very many of them.

A read lock is a memory barrier.  That's why I'm opposed to using non-rcu 
style locking for them.

		-ben
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