Re: [Xen-devel] Question about sharing spinlock_t among VMs in Xen

[Meng Xu <xumengpanda@xxxxxxxxx>]

On Mon, Jun 13, 2016 at 5:17 PM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> On 06/13/2016 04:46 PM, Meng Xu wrote:
>> On Mon, Jun 13, 2016 at 2:28 PM, Boris Ostrovsky
>> <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 06/13/2016 01:43 PM, Meng Xu wrote:
>>>> Hi,
>>>>
>>>> I have a quick question about using the Linux spin_lock() in Xen
>>>> environment to protect some host-wide shared (memory) resource among
>>>> VMs.
>>>>
>>>> *** The question is as follows ***
>>>> Suppose I have two Linux VMs sharing the same spinlock_t lock (through
>>>> the sharing memory) on the same host. Suppose we have one process in
>>>> each VM. Each process uses the linux function spin_lock(&lock) [1] to
>>>> grab & release the lock.
>>>> Will these two processes in the two VMs have race on the shared lock?
>>> You can't do this: depending on which Linux version you use you will
>>> find that kernel uses ticketlocks or qlocks locks which keep track of
>>> who is holding the lock (obviously this information is internal to VM).
>> Yes, we are using the ticketlocks. We expose this information to other
>> VMs as well. That's why I'm guessing some data can be corrupted due to
>> races among VMs that share the same ticketlock information.
>
> I don't think it's a race that is the problem. It's that when holder
> releases a lock it wants to kick the waiter. It can't kick CPU in
> another guest and most likely kicks CPU from its own guest (because
> that's where it assumes the waiter is).

I see. Thank you very much, Boris, for the explanation! I really
appreciate it. :-)

Best Regards,

Meng

-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/

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