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[Xen-ia64-devel] Re: [XenPPC] Re: [Xen-devel] [PATCH] [GNTTAB] expandable grant table

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. 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.

 -- Keir

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