Re: [KVM PATCH v3 3/3] KVM: Fix races in irqfd using new eventfd_kref_get interface

Michael S. Tsirkin
2009-06-22T18:27:20Z

On Mon, Jun 22, 2009 at 02:03:59PM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Mon, Jun 22, 2009 at 01:31:29PM -0400, Gregory Haskins wrote:
> >
> >> Michael S. Tsirkin wrote:
> >>
> >>> On Mon, Jun 22, 2009 at 12:05:57PM -0400, Gregory Haskins wrote:
> >>>
> >>>
> >>>> This patch fixes all known races in irqfd, and paves the way to restore
> >>>> DEASSIGN support. For details of the eventfd races, please see the patch
> >>>> presumably commited immediately prior to this one.
> >>>>
> >>>> In a nutshell, we use eventfd_kref_get/put() to properly manage the
> >>>> lifetime of the underlying eventfd. We also use careful coordination
> >>>> with our workqueue to ensure that all irqfd objects have terminated
> >>>> before we allow kvm to shutdown. The logic used for shutdown walks
> >>>> all open irqfds and releases them. This logic can be generalized in
> >>>> the future to allow a subset of irqfds to be released, thus allowing
> >>>> DEASSIGN support.
> >>>>
> >>>> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> >>>>
> >>>>
> >>> I think this patch is a shade too tricky. Some explanation why below.
> >>>
> >>> But I think irqfd_pop is a good idea.
> >>>
> >>>
> >> Yeah, next we can add something like "irqfd_remove(gsi)" in a similar
> >> way to do DEASSIGN.
> >>
> >>
> >>> Here's an alternative design sketch: add a list of irqfds to be shutdown
> >>> in kvm, and create a single-threaded workqueue. To kill an irqfd, move
> >>> it from list of live irqfds to list of dead irqfds, then schedule work
> >>> on a workqueue that walks this list and kills irqfds.
> >>>
> >>>
> >> Yeah, I actually thought of that too, and I think that will work. But
> >> then I realized flush_schedule_work does the same thing and its much
> >> less code. Perhaps it is also much less clear, too ;) At the very
> >> least, you have made me realize I need to comment better.
> >>
> >
> > Not really, it's impossible to document all races one have thought
> > about and avoided.
> >
>
> Heh, that is a very astute observation.
>
> >
> >>>
> >>>
> >>>> ---
> >>>>
> >>>> virt/kvm/eventfd.c | 144 ++++++++++++++++++++++++++++++++++++++++------------
> >>>> 1 files changed, 110 insertions(+), 34 deletions(-)
> >>>>
> >>>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> >>>> index 9656027..67985cd 100644
> >>>> --- a/virt/kvm/eventfd.c
> >>>> +++ b/virt/kvm/eventfd.c
> >>>> @@ -28,6 +28,7 @@
> >>>> #include <linux/file.h>
> >>>> #include <linux/list.h>
> >>>> #include <linux/eventfd.h>
> >>>> +#include <linux/kref.h>
> >>>>
> >>>> /*
> >>>> * --------------------------------------------------------------------
> >>>> @@ -36,26 +37,68 @@
> >>>> * Credit goes to Avi Kivity for the original idea.
> >>>> * --------------------------------------------------------------------
> >>>> */
> >>>> +
> >>>> +enum {
> >>>> + irqfd_flags_shutdown,
> >>>> +};
> >>>> +
> >>>> struct _irqfd {
> >>>> struct kvm *kvm;
> >>>> + struct kref *eventfd;
> >>>>
> >>>>
> >>> Yay, kref.
> >>>
> >>>
> >>>
> >>>> int gsi;
> >>>> struct list_head list;
> >>>> poll_table pt;
> >>>> wait_queue_head_t *wqh;
> >>>> wait_queue_t wait;
> >>>> - struct work_struct inject;
> >>>> + struct work_struct work;
> >>>> + unsigned long flags;
> >>>>
> >>>>
> >>> Just make it "int shutdown"?
> >>>
> >>>
> >> Yep, that is probably fine but we will have to use an explicit wmb in
> >> lieu of a set_bit operation. NBD.
> >>
> >>
> >>>
> >>>
> >>>> };
> >>>>
> >>>> static void
> >>>> -irqfd_inject(struct work_struct *work)
> >>>> +irqfd_release(struct _irqfd *irqfd)
> >>>> +{
> >>>> + eventfd_kref_put(irqfd->eventfd);
> >>>> + kfree(irqfd);
> >>>> +}
> >>>> +
> >>>> +static void
> >>>> +irqfd_work(struct work_struct *work)
> >>>> {
> >>>> - struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
> >>>> + struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
> >>>> struct kvm *kvm = irqfd->kvm;
> >>>>
> >>>> - mutex_lock(&kvm->irq_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);
> >>>> - mutex_unlock(&kvm->irq_lock);
> >>>> + if (!test_bit(irqfd_flags_shutdown, &irqfd->flags)) {
> >>>>
> >>>>
> >>> Why is it safe to test this bit outside of any lock?
> >>>
> >>>
> >> Because the ordering is guaranteed to set_bit(), schedule_work(). All
> >> we need to do is make sure that the work-queue runs at least one more
> >> time after the flag has been set. (Of course, I could have screwed up
> >> too, but that was my rationale).
> >>
> >>
> >>>
> >>>
> >>>> + /* Inject an interrupt */
> >>>> + mutex_lock(&kvm->irq_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);
> >>>> + mutex_unlock(&kvm->irq_lock);
> >>>> + } else {
> >>>>
> >>>>
> >>> Not much shared code here - create a separate showdown work struct?
> >>> They are cheap ...
> >>>
> >>>
> >> We can't because we need to ensure that all inject-jobs complete before
> >> release-jobs. Reading the work-queue code, it would be a deadlock for
> >> the release-job to do a flush_work(inject-job). Therefore, both
> >> workloads are encapsulated into a single job, and we ensure that the job
> >> is launched at least one more time after the flag has been set.
> >>
> >
> > AFAIK schedule_work does not give you in-order guarantees - it's
> > multithreaded. you will have to create a single-threaded workqueue
> > if you want in order execution.
> >
>
> Right, that was my understanding as well. Thats why I do both tasks
> from a single work-item ;)

I don't think this gives ordering guarantees. If the work is already
running on CPU1 and you do schedule it will start running on CPU2.

> >
> >> Of course, now that I wrote that, I realize it was clear-as-mud in the
> >> code and needs some commenting ;)
> >>
> >>
> >>>
> >>>
> >>>> + /* shutdown the irqfd */
> >>>> + struct _irqfd *_irqfd = NULL;
> >>>> +
> >>>> + mutex_lock(&kvm->lock);
> >>>> +
> >>>> + if (!list_empty(&irqfd->list))
> >>>> + _irqfd = irqfd;
> >>>> +
> >>>> + if (_irqfd)
> >>>> + list_del(&_irqfd->list);
> >>>> +
> >>>> + mutex_unlock(&kvm->lock);
> >>>> +
> >>>> + /*
> >>>> + * If the item is not currently on the irqfds list, we know
> >>>> + * we are running concurrently with the KVM side trying to
> >>>> + * remove this item as well.
> >>>>
> >>>>
> >>> We do? How? As far as I can see list is only empty after it has been
> >>> created. Generally, it would be better to either use a flag or use
> >>> list_empty as an indication of going down, but not both.
> >>>
> >>>
> >> I think you are mis-reading that. list_empty(&irqfd->list) is the
> >> individual irqfd list-item, not the kvm->irqfds list itself. This
> >> conditional is telling us whether the irqfd in question is on or off the
> >> list (its effectively an irqfd-specific flag), not whether the global
> >> list is empty. Again, poor commenting on my part.
> >>
> >
> > Yes, but you do INIT_LIST_HEAD in a single place. Once you add
> > irqfd->list to a list, it won't be empty until you init it again.
> >
>
> Good point. I need list_del_init() and then it would work, right?

Hmm, maybe.

> >
> >>>
> >>>
> >>>> Since the KVM side should be
> >>>> + * holding the reference now, and will block behind a
> >>>> + * flush_work(), lets just let them do the release() for us
> >>>> + */
> >>>> + if (!_irqfd)
> >>>> + return;
> >>>> +
> >>>> + irqfd_release(_irqfd);
> >>>> + }
> >>>> }
> >>>>
> >>>> static int
> >>>> @@ -65,25 +108,20 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> >>>> unsigned long flags = (unsigned long)key;
> >>>>
> >>>> /*
> >>>> - * Assume we will be called with interrupts disabled
> >>>> + * called with interrupts disabled
> >>>> */
> >>>> - if (flags & POLLIN)
> >>>> - /*
> >>>> - * Defer the IRQ injection until later since we need to
> >>>> - * acquire the kvm->lock to do so.
> >>>> - */
> >>>> - schedule_work(&irqfd->inject);
> >>>> -
> >>>> if (flags & POLLHUP) {
> >>>> /*
> >>>> - * for now, just remove ourselves from the list and let
> >>>> - * the rest dangle. We will fix this up later once
> >>>> - * the races in eventfd are fixed
> >>>> + * ordering is important: shutdown flag must be visible
> >>>> + * before we schedule
> >>>> */
> >>>> __remove_wait_queue(irqfd->wqh, &irqfd->wait);
> >>>> - irqfd->wqh = NULL;
> >>>> + set_bit(irqfd_flags_shutdown, &irqfd->flags);
> >>>>
> >>>>
> >>> So what happens if a previously scheduled work runs on irqfd
> >>> and sees this flag?
> >>>
> >> My original thought was "thats ok", but now that you mention it I am not
> >> so sure. Ill give it some more thought because maybe you are on to
> >> something.
> >>
> >>
> >>> And note that multiple works can run on irqfd
> >>> in parallel.
> >>>
> >>>
> >> They can? I thought work-queue items were guaranteed to only schedule
> >> once? If what you say is true, its broken, I agree, and Ill need to
> >> revisit. Let me get back to you.
> >>
> >>>
> >>>
> >>>> }
> >>>>
> >>>> + if (flags & (POLLHUP | POLLIN))
> >>>> + schedule_work(&irqfd->work);
> >>>> +
> >>>> return 0;
> >>>> }
> >>>>
> >>>> @@ -102,6 +140,7 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> >>>> {
> >>>> struct _irqfd *irqfd;
> >>>> struct file *file = NULL;
> >>>> + struct kref *kref = NULL;
> >>>> int ret;
> >>>> unsigned int events;
> >>>>
> >>>> @@ -112,7 +151,7 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> >>>> irqfd->kvm = kvm;
> >>>> irqfd->gsi = gsi;
> >>>> INIT_LIST_HEAD(&irqfd->list);
> >>>> - INIT_WORK(&irqfd->inject, irqfd_inject);
> >>>> + INIT_WORK(&irqfd->work, irqfd_work);
> >>>>
> >>>> file = eventfd_fget(fd);
> >>>> if (IS_ERR(file)) {
> >>>> @@ -133,11 +172,13 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> >>>> list_add_tail(&irqfd->list, &kvm->irqfds);
> >>>> mutex_unlock(&kvm->lock);
> >>>>
> >>>> - /*
> >>>> - * Check if there was an event already queued
> >>>> - */
> >>>> - if (events & POLLIN)
> >>>> - schedule_work(&irqfd->inject);
> >>>> + kref = eventfd_kref_get(file);
> >>>> + if (IS_ERR(file)) {
> >>>> + ret = PTR_ERR(file);
> >>>> + goto fail;
> >>>> + }
> >>>> +
> >>>> + irqfd->eventfd = kref;
> >>>>
> >>>> /*
> >>>> * do not drop the file until the irqfd is fully initialized, otherwise
> >>>> @@ -145,9 +186,18 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> >>>> */
> >>>> fput(file);
> >>>>
> >>>> + /*
> >>>> + * Check if there was an event already queued
> >>>> + */
> >>>>
> >>>>
> >>> This comment seems to confuse more that it clarifies:
> >>> queued where? eventfd only counts... Just kill the comment?
> >>>
> >>>
> >>>
> >> non-zero values in eventfd are "queued" as a signal. This test just
> >> checks if an interrupt was already injected before we registered.
> >>
> >
> > After have understood the code I see what you mean, but the comment
> > wasn't helpful and is better left out.
> >
>
> Ok. What if I say "Check if an interrupt is already pending before we
> registered the callback" ;)

Maybe you can say "check if eventfd was already signalled
before we registered the callback"

> >
> >>>> + if (events & POLLIN)
> >>>> + schedule_work(&irqfd->work);
> >>>> +
> >>>> return 0;
> >>>>
> >>>> fail:
> >>>> + if (kref && !IS_ERR(kref))
> >>>> + eventfd_kref_put(kref);
> >>>> +
> >>>> if (file && !IS_ERR(file))
> >>>> fput(file);
> >>>>
> >>>>
> >>> let's add a couple more labels and avoid the kref/file check
> >>> and the initialization above?
> >>>
> >>>
> >> I think that just makes it more confusing, personally. But I will give
> >> it some thought.
> >>
> >>
> >>>
> >>>
> >>>>
> >>>> @@ -161,21 +211,47 @@ kvm_irqfd_init(struct kvm *kvm)
> >>>> INIT_LIST_HEAD(&kvm->irqfds);
> >>>> }
> >>>>
> >>>> +static struct _irqfd *
> >>>> +irqfd_pop(struct kvm *kvm)
> >>>> +{
> >>>> + struct _irqfd *irqfd = NULL;
> >>>> +
> >>>> + mutex_lock(&kvm->lock);
> >>>> +
> >>>> + if (!list_empty(&kvm->irqfds)) {
> >>>> + irqfd = list_first_entry(&kvm->irqfds, struct _irqfd, list);
> >>>> + list_del(&irqfd->list);
> >>>> + }
> >>>> +
> >>>> + mutex_unlock(&kvm->lock);
> >>>> +
> >>>> + return irqfd;
> >>>> +}
> >>>> +
> >>>> void
> >>>> kvm_irqfd_release(struct kvm *kvm)
> >>>> {
> >>>> - struct _irqfd *irqfd, *tmp;
> >>>> + struct _irqfd *irqfd;
> >>>>
> >>>> - list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list) {
> >>>> - if (irqfd->wqh)
> >>>> - remove_wait_queue(irqfd->wqh, &irqfd->wait);
> >>>> + while ((irqfd = irqfd_pop(kvm))) {
> >>>>
> >>>> - flush_work(&irqfd->inject);
> >>>> + remove_wait_queue(irqfd->wqh, &irqfd->wait);
> >>>>
> >>>> - mutex_lock(&kvm->lock);
> >>>> - list_del(&irqfd->list);
> >>>> - mutex_unlock(&kvm->lock);
> >>>> + /*
> >>>> + * We guarantee there will be no more notifications after
> >>>> + * the remove_wait_queue returns. Now lets make sure we
> >>>> + * synchronize behind any outstanding work items before
> >>>> + * releasing the resources
> >>>> + */
> >>>> + flush_work(&irqfd->work);
> >>>>
> >>>> - kfree(irqfd);
> >>>> + irqfd_release(irqfd);
> >>>> }
> >>>> +
> >>>> + /*
> >>>> + * We need to wait in case there are any outstanding work-items
> >>>> + * in flight that had already removed themselves from the list
> >>>> + * prior to entry to this function
> >>>> + */
> >>>>
> >>>>
> >>> Looks scary. Why doesn't the flush above cover all cases?
> >>>
> >>>
> >> The path inside the while() is for when KVM wins the race and finds the
> >> item in the list. It atomically removes it, and is responsible for
> >> freeing it in a coordinated way. In this case, we must block with the
> >> flush_work() before we can irqfd_release() so that we do not yank the
> >> memory out from under a running work-item.
> >>
> >> The flush_scheduled_work() is for when eventfd wins the race and has
> >> already removed itself from the list in the "shutdown" path in the
> >> work-item. We want to make sure that kvm_irqfd_release() cannot return
> >> until all work-items have exited to prevent something like the kvm.ko
> >> module unloading while the work-item is still in flight.
> >> Thanks Michael,
> >> -Greg
> >>
> >>>
> >>>
> >>>> + flush_scheduled_work();
> >>>> }
> >>>>
> >>>>
> >>> --
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