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

Paul E. McKenney
2009-06-02T18:03:18Z

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

> 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);
> +}
> +
> +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
> + */

The lockless references are all harmless even if carried out after the
kvm structure has been removed? Or does there need to be a ->deleted
field that allows the lockless references to ignore a kvm structure that
has already been deleted?

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.

> + synchronize_srcu(&irqfd->srcu);
> + kvm_put_kvm(kvm);
> }
>
> 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.

> + 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.

> 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;
> }
>