Re: [RFC] KVM: x86: conditionally acquire/release slots_lock on entry/exit

[Avi Kivity <avi@redhat.com>]

On 09/14/2009 01:49 AM, Marcelo Tosatti wrote:
>
>>> I think synchronize_srcu() will be deferred until the fault path is
>>> complete (and srcu_read_unlock() runs).  Copying someone who knows for
>>> sure.
>>>        
>> Yes, synchronize_srcu() will block until srcu_read_unlock() in this
>> scenario, assuming that the same srcu_struct is used by both.
>>      
> Right it will. But this does not stop the fault path from creating
> shadow pages with stale sp->gfn (the only way to do that would be mutual
> exclusion AFAICS).
>    

So we put the kvm_mmu_zap_pages() call as part of the synchronize_srcu() 
callback to take advantage of the srcu guarantees.  We know that when 
when the callback is called all new reads see the new slots and all old 
readers have completed.

> The RCU read-protected side does not stop a new memslots pointer from
> being assigned (with rcu_assign_pointer), does it?
>
>    

It doesn't.  It only gives you a point in time where you know no one is 
using the old pointer, but before it has been deleted.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.