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xen-ia64-devel

Re: [XenPPC] Re: [Xen-devel] [PATCH] [GNTTAB] expandable grant table

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [XenPPC] Re: [Xen-devel] [PATCH] [GNTTAB] expandable grant table
From: Hollis Blanchard <hollisb@xxxxxxxxxx>
Date: Mon, 26 Feb 2007 18:05:18 -0600
Cc: xen-ppc-devel@xxxxxxxxxxxxxxxxxxx, Isaku Yamahata <yamahata@xxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxxxxxxxx>, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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Reply-to: Hollis Blanchard <hollisb@xxxxxxxxxx>
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On Mon, 2007-02-26 at 23:39 +0000, Keir Fraser wrote:
> On 26/2/07 23:31, "Hollis Blanchard" <hollisb@xxxxxxxxxx> wrote:
> 
> > 
> > Hi Yamahata-san, thanks very much for your patch!
> > 
> > I'm confused about one thing though: why do we need to take a lock to
> > read d->grant_table->nr_grant_frames? It's a simple integer, so no lock
> > is required or useful.
> 
> I haven't got the ppc code immediately to hand but the x86 code will
> dynamically grow the grant table based on requests made via the
> add_to_physmap hypercall, hence it needs to take the lock.

OK, I see x86's XENMAPSPACE_grant_table handler; the locking makes sense
there.

> I see ia64 takes
> it also even though it only reads from nr_grant_frames (it won;t dynamically
> expand via the add_to_physmap hypercall). That's possibly overkill although
> there is some concern over reading nr_grant_frames versus looking up a
> grant-table page that appears within the range [0, nr_grant_frames-1] which
> taking the lock trivially sidesteps. If ia64 didn't take the lock it might
> be possible to see an increased value of nr_grant_frames that the expand
> function hadn't yet fully installed the new grant-table pages for yet.

I don't believe that's a concern, since updating
grant_table->nr_grant_frames is the very last step in
gnttab_grow_table(), and it will only grow.

The comments about locking around nr_grant_frames() and
nr_grant_entries() are confusing at best, since you don't need a lock to
read an integer...

-- 
Hollis Blanchard
IBM Linux Technology Center


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