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SubjectRe: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle task
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On 01/08/2017 13:13, Wanpeng Li wrote:
> 2017-08-01 18:37 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>> On 27/07/2017 11:05, Wanpeng Li wrote:
>>> Commit 9b132fbe5419 (Add rcu user eqs exception hooks for async page fault)
>>> adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu idle eqs
>>> when needed, to protect the code that needs use rcu. There is no need to call
>>> this pairs if async page fault is not triggered from idle task.
>>
>> This is true, but I think the patch is making things more complex where
>> it could be simplifying them. Right now, the "native_safe_halt" branch
>> is calling rcu_irq_exit/enter but the "schedule" branch is not. Could
>> you just pull rcu_irq_exit/enter outside the "if", so that you inform
>> the RCU subsystem even in the !n.halted case?
>
> How about something like this?

If it works, it's perfect. :)

Please add Cc for stable kernels too in v4.

Paolo

> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 71c17a5..d04e30e 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -151,6 +151,8 @@ void kvm_async_pf_task_wait(u32 token)
> if (hlist_unhashed(&n.link))
> break;
>
> + rcu_irq_exit();
> +
> if (!n.halted) {
> local_irq_enable();
> schedule();
> @@ -159,11 +161,11 @@ void kvm_async_pf_task_wait(u32 token)
> /*
> * We cannot reschedule. So halt.
> */
> - rcu_irq_exit();
> native_safe_halt();
> local_irq_disable();
> - rcu_irq_enter();
> }
> +
> + rcu_irq_enter();
> }
> if (!n.halted)
> finish_swait(&n.wq, &wait);
>
> Regards,
> Wanpeng Li
>

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