Re: [Xen-devel] [PATCH] Add RCU support into Xen - Repost

To:	"Santos, Jose Renato G" <joserenato.santos@xxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject:	Re: [Xen-devel] [PATCH] Add RCU support into Xen - Repost
From:	Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date:	Tue, 30 Jan 2007 20:23:10 +0000
Cc:	"Turner, Yoshio" <yoshio_turner@xxxxxx>, Jose Renato Santos <jsantos@xxxxxxxxxx>, G John Janakiraman <john@xxxxxxxxxxxxxxxxxxx>
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On 30/1/07 7:05 pm, "Santos, Jose Renato G" <joserenato.santos@xxxxxx>
wrote:

>   While doing that I noticed that you removed the definitions of
>   "rcu_read_lock()" and "rcu_read_unlock()" from rcupdate.h.
>   Although they are really NOPs I think it would be good if we use
>   them in all RCU read critical sections, for code clarity
>   and documentation (as linux does).
>   Can we add those back?

This would require some interaction with new find_domain_by_id(). The
invocation of find_domain_by_id() and all subsequent uses of its return
value will need to be contained within an rcu_read_lock()/rcu_read_unlock()
pair which seems kind of a pain. Particularly since this documentation is
weak -- it may not immediately be clear in future what is being protected by
the rcu read-side region (since the lock routines do not take a parameter).
And I'm not sure how the documentation helps: who will it be useful to?

Linux needs them for more than just documentation as it has to disable
preemption (a feature which I don't expect Xen to ever acquire). I don't
know whether it would have them otherwise.

 -- Keir

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