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

Gregory Haskins
2009-06-02T18:23:34Z

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Paul E. McKenney wrote:
> On Tue, Jun 02, 2009 at 11:15:38AM -0400, Gregory Haskins wrote:
>
>> Assigning an irqfd object to a kvm object creates a relationship that we
>> currently manage by having the kvm oject acquire/hold a file* reference to
>> the underlying eventfd. The lifetime of these objects is properly maintained
>> by decoupling the two objects whenever the irqfd is closed or kvm is closed,
>> whichever comes first.
>>
>> However, the irqfd "close" method is less than ideal since it requires two
>> system calls to complete (one for ioctl(kvmfd, IRQFD_DEASSIGN), the other for
>> close(eventfd)). This dual-call approach was utilized because there was no
>> notification mechanism on the eventfd side at the time irqfd was implemented.
>>
>> Recently, Davide proposed a patch to send a POLLHUP wakeup whenever an
>> eventfd is about to close. So we eliminate the IRQFD_DEASSIGN ioctl (*)
>> vector in favor of sensing the desassign automatically when the fd is closed.
>> The resulting code is slightly more complex as a result since we need to
>> allow either side to sever the relationship independently. We utilize SRCU
>> to guarantee stable concurrent access to the KVM pointer without adding
>> additional atomic operations in the fast path.
>>
>> At minimum, this design should be acked by both Davide and Paul (cc'd).
>>
>> (*) The irqfd patch does not exist in any released tree, so the understanding
>> is that we can alter the irqfd specific ABI without taking the normal
>> precautions, such as CAP bits.
>>
>
> A few questions and suggestions interspersed below.
>
> Thanx, Paul
>

Thanks for the review, Paul.

(FYI: This isn't quite what I was asking you about on IRC yesterday, but
it's related...and the SRCU portion of the conversation *did* inspire me
here. Just note that the stuff I was asking about is still forthcoming)

>
>> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
>> CC: Davide Libenzi <davidel@xmailserver.org>
>> CC: Michael S. Tsirkin <mst@redhat.com>
>> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> ---
>>
>> include/linux/kvm.h | 2 -
>> virt/kvm/eventfd.c | 177 +++++++++++++++++++++++----------------------------
>> virt/kvm/kvm_main.c | 3 +
>> 3 files changed, 81 insertions(+), 101 deletions(-)
>>
>> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
>> index 632a856..29b62cc 100644
>> --- a/include/linux/kvm.h
>> +++ b/include/linux/kvm.h
>> @@ -482,8 +482,6 @@ struct kvm_x86_mce {
>> };
>> #endif
>>
>> -#define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
>> -
>> struct kvm_irqfd {
>> __u32 fd;
>> __u32 gsi;
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> index f3f2ea1..6ed62e2 100644
>> --- a/virt/kvm/eventfd.c
>> +++ b/virt/kvm/eventfd.c
>> @@ -37,26 +37,63 @@
>> * --------------------------------------------------------------------
>> */
>> struct _irqfd {
>> + struct mutex lock;
>> + struct srcu_struct srcu;
>> struct kvm *kvm;
>> int gsi;
>> - struct file *file;
>> struct list_head list;
>> poll_table pt;
>> wait_queue_head_t *wqh;
>> wait_queue_t wait;
>> - struct work_struct work;
>> + struct work_struct inject;
>> };
>>
>> static void
>> irqfd_inject(struct work_struct *work)
>> {
>> - struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
>> - struct kvm *kvm = irqfd->kvm;
>> + struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
>> + struct kvm *kvm;
>> + int idx;
>> +
>> + idx = srcu_read_lock(&irqfd->srcu);
>> +
>> + kvm = rcu_dereference(irqfd->kvm);
>> + if (kvm) {
>> + 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);
>> + mutex_unlock(&kvm->lock);
>> + }
>> +
>> + srcu_read_unlock(&irqfd->srcu, idx);
>> +}
>>

[1]

>> +
>> +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);
>>

[2]

>> +
>> + 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
>> + */
>>
>
> The lockless references are all harmless even if carried out after the
> kvm structure has been removed?

No, but all ([1]) references to my knowledge occur within an srcu
read-side CS, and we only drop the object reference ([3]) outside of
that CS by virtue of the synchronize_srcu() barrier below. The one
notable exception is [2], which I don't declare as a read-side CS since
I hold the mutex during the swap.

OTOH, this is really my _intention_, not _reality_ per se. ;) E.g. I
may have completely flubbed up the design, so I'm glad you are looking
at it.

> Or does there need to be a ->deleted
> field that allows the lockless references to ignore a kvm structure that
> has already been deleted?
>

I guess you could say that the "rcu_assign_pointer(NULL)" is my
"deleted" field. The reader-side code in question should check for that
condition before proceeding.

> On the other hand, if it really is somehow OK for kvm_set_irq() to be
> called on an already-deleted kvm structure, then this code would be OK
> as is.
>

Definitely not, so if you think that can happen please raise the flag.
>
>> + synchronize_srcu(&irqfd->srcu);
>> + kvm_put_kvm(kvm);
>>

[3]

>> }
>>
>> static int
>> @@ -64,12 +101,28 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>> {
>> struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
>>
>> - /*
>> - * 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);
>> + switch ((unsigned long)key) {
>> + case 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);
>> + break;
>> + case POLLHUP:
>> + /*
>> + * The POLLHUP is called unlocked, so it theoretically should
>> + * be safe to remove ourselves from the wqh
>> + */
>> + remove_wait_queue(irqfd->wqh, &irqfd->wait);
>> + flush_work(&irqfd->inject);
>> + irqfd_disconnect(irqfd);
>>
>
> Good. The fact that irqfd_disconnect() does a synchronize_srcu()
> prevents any readers from trying to do an SRCU operation on an already
> cleaned-up srcu_struct, so this does look safe to me.
>

As an additional data point, we can guarantee that we will never be
called again since we synchronously unhook from the wqh and kvm->irqfds
list, and the POLLHUP is called from f_ops->release().

>
>> + cleanup_srcu_struct(&irqfd->srcu);
>> + kfree(irqfd);
>> + break;
>> + }
>>
>> return 0;
>> }
>> @@ -84,8 +137,8 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
>> add_wait_queue(wqh, &irqfd->wait);
>> }
>>
>> -static int
>> -kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
>> +int
>> +kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
>> {
>> struct _irqfd *irqfd;
>> struct file *file = NULL;
>> @@ -95,10 +148,12 @@ kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
>> if (!irqfd)
>> return -ENOMEM;
>>
>> + mutex_init(&irqfd->lock);
>> + init_srcu_struct(&irqfd->srcu);
>> irqfd->kvm = kvm;
>> irqfd->gsi = gsi;
>> INIT_LIST_HEAD(&irqfd->list);
>> - INIT_WORK(&irqfd->work, irqfd_inject);
>> + INIT_WORK(&irqfd->inject, irqfd_inject);
>>
>> /*
>> * Embed the file* lifetime in the irqfd.
>> @@ -120,12 +175,18 @@ kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
>> if (ret < 0)
>> goto fail;
>>
>> - irqfd->file = file;
>> + kvm_get_kvm(kvm);
>>
>> mutex_lock(&kvm->lock);
>> list_add_tail(&irqfd->list, &kvm->irqfds);
>>
>
> Doesn't the above need to be list_add_tail_rcu()? Unless I am confused,
> this is the point at which the new SRCU-protected structure is first
> made public. If so, you really do need list_add_tail_rcu() to make sure
> that concurrent readers don't see pre-initialized values in the structure.
>

I *think* this is ok as a traditional list, because the only paths that
touch this list are guarded by the kvm->lock mutex. Let me know if you
see otherwise or if that is not enough.
>
>> mutex_unlock(&kvm->lock);
>>
>> + /*
>> + * do not drop the file until the irqfd is fully initialized, otherwise
>> + * we might race against the POLLHUP
>> + */
>> + fput(file);
>> +
>> return 0;
>>
>> fail:
>> @@ -139,97 +200,17 @@ fail:
>> return ret;
>> }
>>
>> -static void
>> -irqfd_release(struct _irqfd *irqfd)
>> -{
>> - /*
>> - * The ordering is important. We must remove ourselves from the wqh
>> - * first to ensure no more event callbacks are issued, and then flush
>> - * any previously scheduled work prior to freeing the memory
>> - */
>> - remove_wait_queue(irqfd->wqh, &irqfd->wait);
>> -
>> - flush_work(&irqfd->work);
>> -
>> - fput(irqfd->file);
>> - kfree(irqfd);
>> -}
>> -
>> -static struct _irqfd *
>> -irqfd_remove(struct kvm *kvm, struct file *file, int gsi)
>> -{
>> - struct _irqfd *irqfd;
>> -
>> - mutex_lock(&kvm->lock);
>> -
>> - /*
>> - * linear search isn't brilliant, but this should be an infrequent
>> - * slow-path operation, and the list should not grow very large
>> - */
>> - list_for_each_entry(irqfd, &kvm->irqfds, list) {
>> - if (irqfd->file != file || irqfd->gsi != gsi)
>> - continue;
>> -
>> - list_del(&irqfd->list);
>> - mutex_unlock(&kvm->lock);
>> -
>> - return irqfd;
>> - }
>> -
>> - mutex_unlock(&kvm->lock);
>> -
>> - return NULL;
>> -}
>> -
>> -static int
>> -kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
>> -{
>> - struct _irqfd *irqfd;
>> - struct file *file;
>> - int count = 0;
>> -
>> - file = fget(fd);
>> - if (IS_ERR(file))
>> - return PTR_ERR(file);
>> -
>> - while ((irqfd = irqfd_remove(kvm, file, gsi))) {
>> - /*
>> - * We remove the item from the list under the lock, but we
>> - * free it outside the lock to avoid deadlocking with the
>> - * flush_work and the work_item taking the lock
>> - */
>> - irqfd_release(irqfd);
>> - count++;
>> - }
>> -
>> - fput(file);
>> -
>> - return count ? count : -ENOENT;
>> -}
>> -
>> void
>> kvm_irqfd_init(struct kvm *kvm)
>> {
>> INIT_LIST_HEAD(&kvm->irqfds);
>> }
>>
>> -int
>> -kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
>> -{
>> - if (flags & KVM_IRQFD_FLAG_DEASSIGN)
>> - return kvm_deassign_irqfd(kvm, fd, gsi);
>> -
>> - return kvm_assign_irqfd(kvm, fd, gsi);
>> -}
>> -
>> void
>> kvm_irqfd_release(struct kvm *kvm)
>> {
>> struct _irqfd *irqfd, *tmp;
>>
>> - /* don't bother with the lock..we are shutting down */
>> - list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list) {
>> - list_del(&irqfd->list);
>> - irqfd_release(irqfd);
>> - }
>> + list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list)
>> + irqfd_disconnect(irqfd);
>> }
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 902fed9..a9f62bb 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1029,7 +1029,6 @@ static void kvm_destroy_vm(struct kvm *kvm)
>> spin_lock(&kvm_lock);
>> list_del(&kvm->vm_list);
>> spin_unlock(&kvm_lock);
>> - kvm_irqfd_release(kvm);
>> kvm_free_irq_routing(kvm);
>> kvm_io_bus_destroy(&kvm->pio_bus);
>> kvm_io_bus_destroy(&kvm->mmio_bus);
>> @@ -1064,6 +1063,8 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
>> {
>> struct kvm *kvm = filp->private_data;
>>
>> + kvm_irqfd_release(kvm);
>> +
>> kvm_put_kvm(kvm);
>> return 0;
>> }
>>
>>
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