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

Gregory Haskins
2009-06-15T12:08:23Z

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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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