Re: [PATCH 0/3] KVM: Make kvm_lock non-raw

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-09-22 11:53, Gleb Natapov wrote:
> On Sun, Sep 22, 2013 at 10:53:14AM +0200, Paolo Bonzini wrote:
>> Il 22/09/2013 09:42, Gleb Natapov ha scritto:
>>> On Mon, Sep 16, 2013 at 04:06:10PM +0200, Paolo Bonzini wrote:
>>>> Paul Gortmaker reported a BUG on preempt-rt kernels, due to taking the
>>>> mmu_lock within the raw kvm_lock in mmu_shrink_scan.  He provided a
>>>> patch that shrunk the kvm_lock critical section so that the mmu_lock
>>>> critical section does not nest with it, but in the end there is no reason
>>>> for the vm_list to be protected by a raw spinlock.  Only manipulations
>>>> of kvm_usage_count and the consequent hardware_enable/disable operations
>>>> are not preemptable.
>>>>
>>>> This small series thus splits the kvm_lock in the "raw" part and the
>>>> "non-raw" part.
>>>>
>>>> Paul, could you please provide your Tested-by?
>>>>
>>> Reviewed-by: Gleb Natapov <gleb@redhat.com>
>>>
>>> But why should it go to stable?
>>
>> It is a regression from before the kvm_lock was made raw.  Secondarily,
> It was made raw in 2.6.39 and commit message claims that it is done for
> -rt sake, why regression was noticed only now?

Probably, the patch is stressed to infrequently. Just checked: the issue
was present from day #1 one, what a shame.

> 
>> it takes a much longer time before a patch hits -rt trees (can even be
>> as much as a year) and this patch does nothing on non-rt trees.  So
>> without putting it into stable it would get no actual coverage.
>>
> The change is not completely trivial, it splits lock. There is no
> obvious problem of course, otherwise you wouldn't send it and I
> would ack it :), but it does not mean that the chance for problem is
> zero, so why risk stability of stable even a little bit if the patch
> does not fix anything in stable?
> 
> I do not know how -rt development goes and how it affects decisions for
> stable acceptance, why can't they carry the patch in their tree until
> they move to 3.12?

I think it would be fair to let stable -rt carry these. -rt requires
more specific patching anyway due to the waitqueue issue Paul reported.
But CC'ing Steven to obtain his view.

Jan

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