Re: Question about sharing spinlock_t among VMs in Xen

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 06/13/2016 04:46 PM, Meng Xu wrote:
> On Mon, Jun 13, 2016 at 2:28 PM, Boris Ostrovsky
> <boris.ostrovsky@oracle.com> 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).


-boris



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