From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ross Zwisler <ross.zwisler@linux.intel.com>
Subject: Re: suspicious RCU usage
Date: Mon, 6 Mar 2017 14:04:34 -0700
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 <87ebb08e-944d-6305-c60e-2f062f1876ef@redhat.com>
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Cc: Ross Zwisler <ross.zwisler@linux.intel.com>,
        Xiao Guangrong <guangrong.xiao@linux.intel.com>,
        kvm@vger.kernel.org, Dan Williams <dan.j.williams@intel.com>
To: Paolo Bonzini <pbonzini@redhat.com>
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On Tue, Feb 28, 2017 at 12:06:21PM +0100, Paolo Bonzini wrote:
> I think it's just a trivial ordering bug:
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 14f65a5f938e..5215629ad692 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -161,8 +161,8 @@ void kvm_async_pf_task_wait(u32 token)
>  			 */
>  			rcu_irq_exit();
>  			native_safe_halt();
> -			rcu_irq_enter();
>  			local_irq_disable();
> +			rcu_irq_enter();
>  		}
>  	}
>  	if (!n.halted)
> 
> 
> because the async page fault code checks for preempt_disable() already,
> and kmap_atomic already does preempt_disable outside pagefault_disable.
> 
> Let me know if the above is enough to fix it for you.

Well, unfortunately I can't reproduce the issue with enough reliability to say
whether or not the fix is making a difference. :(

FWIW, I did re-run my tests with this fix applied, and no new errors cropped
up.

- Ross