linux-kernel - Re: [PATCH] KVM: async_pf: Fix rcu_irq

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Date:   Wed, 7 Jun 2017 15:05:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: async_pf: Fix rcu_irq_enter/exit() usage



On 07/06/2017 14:04, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> 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@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  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;
>  }