Re: [PATCH 2/2] KVM: Convert read-only users of vm_list to RCU

[Avi Kivity <avi@redhat.com>]

On 02/10/2011 04:34 PM, Jan Kiszka wrote:
> On 2011-02-10 15:26, Avi Kivity wrote:
> >  On 02/10/2011 03:47 PM, Jan Kiszka wrote:
> >>>>
> >>>>   Accept for mmu_shrink, which is write but not delete, thus works without
> >>>>   that slow synchronize_rcu.
> >>>
> >>>   I don't really see how you can implement list_move_rcu(), it has to be
> >>>   atomic or other users will see a partial vm_list.
> >>
> >>  Right, even if we synchronized that step cleanly, rcu-protected users
> >>  could miss the moving vm during concurrent list walks.
> >>
> >>  What about using a separate mutex for protecting vm_list instead?
> >>  Unless I missed some detail, mmu_shrink should allow blocking.
> >
> >  What else does kvm_lock protect?
>
> Someone tried to write a locking.txt and stated that it's also
> protecting enabling/disabling hardware virtualization. But that guy may
> have overlooked something.

Right.  I guess splitting that lock makes sense.

> >
> >  I think we could simply reduce the amount of time we hold kvm_lock.
> >  Pick a vm, ref it, list_move_tail(), unlock, then do the actual
> >  shrinking.  Of course taking a ref must be done carefully, we might
> >  already be in kvm_destroy_vm() at that time.
> >
>
> Plain mutex held across the whole mmu_shrink loop is still simpler and
> should be sufficient - unless we also have to deal with scalability
> issues if that handler is able to run concurrently. But based on how we
> were using kvm_lock so far...

I don't think a mutex would work for kvmclock_cpufreq_notifier().  At 
the very least, we'd need a preempt_disable() there.  At the worst, the 
notifier won't like sleeping.

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