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

Jan Kiszka
2011-02-10T13:47:54Z

On 2011-02-10 14:19, Avi Kivity wrote:
> On 02/10/2011 03:14 PM, Jan Kiszka wrote:
>> On 2011-02-10 13:57, Avi Kivity wrote:
>>> On 02/10/2011 02:56 PM, Avi Kivity wrote:
>>>>> What's the benefit? The downside is a bit more complexity as you need an
>>>>> additional callback handler.
>>>>
>>>>
>>>> synchronize_rcu() can be very slow (its a systemwide operation), and
>>>> mmu_shrink() can be called often on a loaded system.
>>>>
>>>
>>> In fact this just shows that vm_list is not a good candidate for rcu;
>>> rcu is useful where most operations are reads, but if we discount stats,
>>> most operations on vm_list are going to be writes.
>>
>> 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.

Jan

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