linux-kernel - Re: [RFC PATCH v3] Add rcu user eqs exception hooks for async page fault

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Message-ID: <20121130102652.GG20873@redhat.com>
Date:	Fri, 30 Nov 2012 12:26:52 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-next list <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	sasha.levin@...cle.com, avi@...hat.com
Subject: Re: [RFC PATCH v3] Add rcu user eqs exception hooks for async page
 fault

On Fri, Nov 30, 2012 at 05:18:41PM +0800, Li Zhong wrote:
> This patch adds user eqs exception hooks for async page fault page not
> present code path, to exit the user eqs and re-enter it as necessary. 
> 
> Async page fault is different from other exceptions that it may be
> triggered from idle process, so we still need rcu_irq_enter() and
> rcu_irq_exit() to exit cpu idle eqs when needed, to protect the code
> that needs use rcu.
> 
> As Frederic pointed out it would be safest and simplest to protect the
> whole kvm_async_pf_task_wait(). Otherwise, "we need to check all the
> code there deeply for potential RCU uses and ensure it will never be
> extended later to use RCU.".
> 
> However, We'd better re-enter the cpu idle eqs if we get the exception
> in cpu idle eqs, by calling rcu_irq_exit() before native_safe_halt(). 
> 
> So the patch does what Frederic suggested for rcu_irq_*() API usage
> here, except that I moved the rcu_irq_*() pair originally in
> do_async_page_fault() into kvm_async_pf_task_wait(). 
> 
> That's because, I think it's better to have rcu_irq_*() pairs to be in
> one function ( rcu_irq_exit() after rcu_irq_enter() ), especially here,
> kvm_async_pf_task_wait() has other callers, which might cause
> rcu_irq_exit() be called without a matching rcu_irq_enter() before it,
> which is illegal if the cpu happens to be in rcu idle state. 
> 
> Signed-off-by: Li Zhong <zhong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kernel/kvm.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 4180a87..342b00b 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -42,6 +42,7 @@
>  #include <asm/apic.h>
>  #include <asm/apicdef.h>
>  #include <asm/hypervisor.h>
> +#include <asm/rcu.h>
>  
>  static int kvmapf = 1;
>  
> @@ -112,6 +113,8 @@ void kvm_async_pf_task_wait(u32 token)
>  	DEFINE_WAIT(wait);
>  	int cpu, idle;
>  
> +	rcu_irq_enter();
> +
Why move rcu_irq_*() calls into kvm_async_pf_task_wait()?

>  	cpu = get_cpu();
>  	idle = idle_cpu(cpu);
>  	put_cpu();
> @@ -123,6 +126,8 @@ void kvm_async_pf_task_wait(u32 token)
>  		hlist_del(&e->link);
>  		kfree(e);
>  		spin_unlock(&b->lock);
> +
> +		rcu_irq_exit();
We can skip that if  rcu_irq_*() will stay outside.

>  		return;
>  	}
>  
> @@ -147,13 +152,16 @@ void kvm_async_pf_task_wait(u32 token)
>  			/*
>  			 * We cannot reschedule. So halt.
>  			 */
> +			rcu_irq_exit();
>  			native_safe_halt();
> +			rcu_irq_enter();
>  			local_irq_disable();
>  		}
>  	}
>  	if (!n.halted)
>  		finish_wait(&n.wq, &wait);
>  
> +	rcu_irq_exit();
>  	return;
>  }
>  EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
> @@ -247,10 +255,10 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
>  		break;
>  	case KVM_PV_REASON_PAGE_NOT_PRESENT:
>  		/* page is swapped out by the host. */
> -		rcu_irq_enter();
> +		exception_enter(regs);
>  		exit_idle();
>  		kvm_async_pf_task_wait((u32)read_cr2());
> -		rcu_irq_exit();
> +		exception_exit(regs);
>  		break;
>  	case KVM_PV_REASON_PAGE_READY:
>  		rcu_irq_enter();
> -- 
> 1.7.11.4

--
			Gleb.
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