Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead of an explicit ioctl

[Gregory Haskins <ghaskins@novell.com>]

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Michael S. Tsirkin wrote:
> On Sun, Jun 14, 2009 at 11:39:21PM -0400, Gregory Haskins wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> On Sun, Jun 14, 2009 at 08:53:11AM -0400, Gregory Haskins wrote:
>>>   
>>>       
>>>> Michael S. Tsirkin wrote:
>>>>     
>>>>         
>>>>> On Thu, Jun 04, 2009 at 08:48:12AM -0400, Gregory Haskins wrote:
>>>>>   
>>>>>       
>>>>>           
>>>>>> +static void
>>>>>> +irqfd_disconnect(struct _irqfd *irqfd)
>>>>>> +{
>>>>>> +	struct kvm *kvm;
>>>>>> +
>>>>>> +	mutex_lock(&irqfd->lock);
>>>>>> +
>>>>>> +	kvm = rcu_dereference(irqfd->kvm);
>>>>>> +	rcu_assign_pointer(irqfd->kvm, NULL);
>>>>>> +
>>>>>> +	mutex_unlock(&irqfd->lock);
>>>>>> +
>>>>>> +	if (!kvm)
>>>>>> +		return;
>>>>>>  
>>>>>>  	mutex_lock(&kvm->lock);
>>>>>> -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
>>>>>> -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
>>>>>> +	list_del(&irqfd->list);
>>>>>>  	mutex_unlock(&kvm->lock);
>>>>>> +
>>>>>> +	/*
>>>>>> +	 * It is important to not drop the kvm reference until the next grace
>>>>>> +	 * period because there might be lockless references in flight up
>>>>>> +	 * until then
>>>>>> +	 */
>>>>>> +	synchronize_srcu(&irqfd->srcu);
>>>>>> +	kvm_put_kvm(kvm);
>>>>>>  }
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> So irqfd object will persist after kvm goes away, until eventfd is closed?
>>>>>   
>>>>>       
>>>>>           
>>>> Yep, by design.  It becomes part of the eventfd and is thus associated
>>>> with its lifetime.  Consider it as if we made our own anon-fd
>>>> implementation for irqfd and the lifetime looks similar.  The difference
>>>> is that we are reusing eventfd and its interface semantics.
>>>>     
>>>>         
>>>>>   
>>>>>       
>>>>>           
>>>>>>  
>>>>>>  static int
>>>>>>  irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>>>>>>  {
>>>>>>  	struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
>>>>>> +	unsigned long flags = (unsigned long)key;
>>>>>>  
>>>>>> -	/*
>>>>>> -	 * The wake_up is called with interrupts disabled.  Therefore we need
>>>>>> -	 * to defer the IRQ injection until later since we need to acquire the
>>>>>> -	 * kvm->lock to do so.
>>>>>> -	 */
>>>>>> -	schedule_work(&irqfd->work);
>>>>>> +	if (flags & POLLIN)
>>>>>> +		/*
>>>>>> +		 * The POLLIN wake_up is called with interrupts disabled.
>>>>>> +		 * Therefore we need to defer the IRQ injection until later
>>>>>> +		 * since we need to acquire the kvm->lock to do so.
>>>>>> +		 */
>>>>>> +		schedule_work(&irqfd->inject);
>>>>>> +
>>>>>> +	if (flags & POLLHUP) {
>>>>>> +		/*
>>>>>> +		 * The POLLHUP is called unlocked, so it theoretically should
>>>>>> +		 * be safe to remove ourselves from the wqh using the locked
>>>>>> +		 * variant of remove_wait_queue()
>>>>>> +		 */
>>>>>> +		remove_wait_queue(irqfd->wqh, &irqfd->wait);
>>>>>> +		flush_work(&irqfd->inject);
>>>>>> +		irqfd_disconnect(irqfd);
>>>>>> +
>>>>>> +		cleanup_srcu_struct(&irqfd->srcu);
>>>>>> +		kfree(irqfd);
>>>>>> +	}
>>>>>>  
>>>>>>  	return 0;
>>>>>>  }
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> And it is removed by this function when eventfd is closed.
>>>>> But what prevents the kvm module from going away, meanwhile?
>>>>>   
>>>>>       
>>>>>           
>>>> Well, we hold a reference to struct kvm until we call
>>>> irqfd_disconnect().  If kvm closes first, we disconnect and disassociate
>>>> all references to kvm leaving irqfd->kvm = NULL.  Likewise, if irqfd
>>>> closes first, we disassociate with kvm with the above quoted logic.  In
>>>> either case, we are holding a kvm reference up until that "disconnect"
>>>> point.  Therefore kvm should not be able to disappear before that
>>>> disconnect, and after that point we do not care.
>>>>     
>>>>         
>>> Yes, we do care.
>>>
>>> Here's the scenario in more detail:
>>>
>>> - kvm is closed
>>> - irq disconnect is called
>>> - kvm is put
>>> - kvm module is removed: all irqs are disconnected
>>> - eventfd closes and triggers callback into removed kvm module
>>> - crash
>>>   
>>>       
>> [ lightbulb turns on]
>>
>> Ah, now I see the point you were making.  I thought you were talking
>> about the .text in kvm_set_irq() (which would be protected by my
>> kvm_get_kvm() reference afaict).  But you are actually talking about the
>> irqfd .text itself.  Indeed, you are correct that is this currently a
>> race.  Good catch!
>>
>>     
>>>   
>>>       
>>>> If that is not sufficient to prevent kvm.ko from going away in the
>>>> middle, then IMO kvm_get_kvm() has a bug, not irqfd. ;) However, I
>>>> believe everything is actually ok here.
>>>>
>>>> -Greg
>>>>
>>>>     
>>>>         
>>> BTW, why can't we remove irqfds in kvm_release?
>>>   
>>>       
>> Well, this would be ideal but we run into that bi-directional reference
>> thing that we talked about earlier and we both agree is non-trivial to
>> solve.  Solving this locking problem would incidentally also pave the
>> way for restoring the DEASSIGN feature, so patches welcome!
>>     
>
> So far the only workable approach that I see is reverting the POLLHUP
> patch. I agree it looks pretty, but DEASSIGN and closing the races is
> more important IMO. And locking will definitely become much simpler.
>
>   
>> In the meantime, I think we can close the hole you found with the
>> following patch (build-tested only):
>>
>> commit f3a8dccc9e815599438e9feb0ea53e8eb10ad2b3
>> Author: Gregory Haskins <ghaskins@novell.com>
>> Date:   Sun Jun 14 23:37:49 2009 -0400
>>
>>     KVM: make irqfd take kvm.ko module reference
>>    
>>     Michael Tsirkin pointed out that we currently have a race between someone
>>     holding an irqfd reference and an rmmod against kvm.ko.  This patch closes
>>     that hole by making sure that irqfd holds a kvm.ko reference for its lifetime.
>>    
>>     Found-by: Michael S. Tsirkin <mst@redhat.com>
>>     Signed-off-by: Gregory Haskins <ghaskins@novell.com>
>>
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> index 2c8028c..67e4eca 100644
>> --- a/virt/kvm/eventfd.c
>> +++ b/virt/kvm/eventfd.c
>> @@ -29,6 +29,7 @@
>>  #include <linux/list.h>
>>  #include <linux/eventfd.h>
>>  #include <linux/srcu.h>
>> +#include <linux/module.h>
>>  
>>  /*
>>   * --------------------------------------------------------------------
>> @@ -123,6 +124,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int
>> sync, void
>>  *key)
>>  
>>                 cleanup_srcu_struct(&irqfd->srcu);
>>                 kfree(irqfd);
>> +               module_put(THIS_MODULE);
>>         }
>>  
>>         return 0;
>>     
>
> module_put(THIS_MODULE) is always a bug unless you know that someone has
> a reference to the current module: the module could go away between this
> call and returning from function.
>   

Hmm.  I understand what you are saying conceptually (i.e. the .text
could get yanked before we hit the next line of code, in this case the
"return 0").  However, holding a reference when you _know_ someone else
holds a reference to me says that one of the references is redundant. 
In addition, there is certainly plenty of precedence for
module_put(THIS_MODULE) all throughout the kernel (including
module_put_and_exit()).  Are those broken as well?

In any case, one of the patches I have in queue to push to Davide for
eventfd may provide a good solution to this problem as well, so I will
get that polished up today.

Thanks Michael,
-Greg

>   
>> @@ -176,6 +178,7 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
>>         if (ret < 0)
>>                 goto fail;
>>  
>> +       __module_get(THIS_MODULE);
>>         kvm_get_kvm(kvm);
>>  
>>         mutex_lock(&kvm->lock);
>>
>>
>>
>>     
>
>
>   



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