[Xen-devel] Re: [PATCH] mem_sharing: fix race condition of nominate and unshare

[Jui-Hao Chiang <juihaochiang@xxxxxxxxx>]

Hi, Tim:

On Sat, Jan 8, 2011 at 12:09 AM, Tim Deegan <Tim.Deegan@xxxxxxxxxx> wrote:

At 06:02 +0000 on 07 Jan (1294380120), Jui-Hao Chiang wrote:
> One of the solution is to
> (a) Simply replace shr_lock with p2m_lock.

I think this is the best choice.   If we find that the p2m lock is a
bottleneck we can address it later.

Just to be skeptic.
Why doesn't mfn_to_gfn() take p2m lock when querying the p2m type? Is there any quarantee that the resulting type is correct and trustful?
For example:
(1) User1 query the p2m type:
mfn_to_gfn(...&p2mt);
if (p2mt == p2m_ram_rw) /* do something based on the p2m type result? */

(2) User2 modify the p2m type
p2m_lock(p2m);
set_p2m_entry(..... p2m_ram_rw);
p2m_unlock(p2m);

Thanks,
Jui-Hao

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