Re: [PATCH] KVM: async_pf: Fix rcu_irq_enter/exit() usage

From: Paolo Bonzini
Date: Wed Jun 07 2017 - 09:05:27 EST




On 07/06/2017 14:04, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> 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 patch fixes it by rcu irq exit if it is not triggered from idle task to
> avoid rcu hang after schedule() in the for loop.

How does the bug manifest?

Paolo

>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> arch/x86/kernel/kvm.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 43e10d6..e70ed72 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -141,6 +141,8 @@ void kvm_async_pf_task_wait(u32 token)
> n.token = token;
> n.cpu = smp_processor_id();
> n.halted = is_idle_task(current) || preempt_count() > 1;
> + if (!n.halted)
> + rcu_irq_exit();
> init_swait_queue_head(&n.wq);
> hlist_add_head(&n.link, &b->list);
> raw_spin_unlock(&b->lock);
> @@ -167,8 +169,9 @@ void kvm_async_pf_task_wait(u32 token)
> }
> if (!n.halted)
> finish_swait(&n.wq, &wait);
> + else
> + rcu_irq_exit();
>
> - rcu_irq_exit();
> return;
> }