Re: [RFC PATCH v3] Add rcu user eqs exception hooks for async pagefault

[Gleb Natapov]

On Mon, Dec 03, 2012 at 10:08:32AM +0800, Li Zhong wrote:
> On Fri, 2012-11-30 at 12:26 +0200, Gleb Natapov wrote:
> > 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@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >  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()?
> 
> I think it is not good for a function to have a rcu_irq_exit(), which
> needs a matching rcu_irq_enter() in its caller. 
> 
> Here, if not move rcu_irq_* in, then the rcu_irq_exit() before
> native_safe_halt() in kvm_async_pf_task_wait() is the one that needs the
> matching rcu_irq_enter() in do_async_page_fault(). And, for this case,
> kvm_async_pf_task_wait() even has other caller - pf_interception().
> Maybe it will always be rcu non-idle for pf_interception (so a matching
> rcu_irq_enter() is not needed), or maybe we could (or need) add
> rcu_irq_*() in pf_interception().  But I still think it's good to have
> those function calls that need to be matched be contained in one
> function.
> 
kvm_async_pf_task_wait() call from pf_interception() will always go to
schedule() path. I get your point and am fine with the patch as is.

> Thanks, Zhong
> 
> > >  	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.
> > 
> 

--
			Gleb.
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