Re: [Xen-devel] [PATCH v12 2/3] x86/mem_sharing: reset a fork

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.03.2020 15:48, Tamas K Lengyel wrote:
> On Thu, Mar 26, 2020 at 4:17 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 23.03.2020 18:04, Tamas K Lengyel wrote:
>>> +static int mem_sharing_fork_reset(struct domain *d, struct domain *pd)
>>> +{
>>> +    int rc;
>>> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
>>> +    struct page_info *page, *tmp;
>>> +
>>> +    spin_lock(&d->page_alloc_lock);
>>> +    domain_pause(d);
>>
>> Why do you take the lock first?
> 
> No particular reason - does the order matter?

I think you'd better avoid holding a lock for extended periods
of time. And what's perhaps worse, what if a vCPU of the domain
sits in Xen trying to acquire this lock - you'd deadlock trying
to pause the domain then.

Jan