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Subject[KVM PATCH v7 3/5] KVM: Fix races in irqfd using new eventfd_kref_get interface
Date
eventfd currently emits a POLLHUP wakeup on f_ops->release() to generate a
"release" callback. This lets eventfd clients know if the eventfd is about
to go away and is very useful particularly for in-kernel clients. However,
until recently it is not possible to use this feature of eventfd in a
race-free way. This patch utilizes a new eventfd interface to rectify
the problem.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

include/linux/kvm_host.h | 5 +-
virt/kvm/eventfd.c | 153 ++++++++++++++++++++++++++++++++++++++--------
2 files changed, 129 insertions(+), 29 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2451f48..5a90c6a 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -141,7 +141,10 @@ struct kvm {
struct kvm_io_bus mmio_bus;
struct kvm_io_bus pio_bus;
#ifdef CONFIG_HAVE_KVM_EVENTFD
- struct list_head irqfds;
+ struct {
+ spinlock_t lock;
+ struct list_head items;
+ } irqfds;
#endif
struct kvm_vm_stat stat;
struct kvm_arch arch;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 9656027..cb29a85 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -36,16 +36,22 @@
* Credit goes to Avi Kivity for the original idea.
* --------------------------------------------------------------------
*/
+
struct _irqfd {
struct kvm *kvm;
+ struct eventfd_ctx *eventfd;
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 shutdown;
+ int active:1; /* guarded by kvm->irqfds.lock */
};

+static struct workqueue_struct *irqfd_cleanup_wq;
+
static void
irqfd_inject(struct work_struct *work)
{
@@ -58,30 +64,78 @@ irqfd_inject(struct work_struct *work)
mutex_unlock(&kvm->irq_lock);
}

+/*
+ * Race-free decouple logic (ordering is critical)
+ */
+static void
+irqfd_shutdown(struct work_struct *work)
+{
+ struct _irqfd *irqfd = container_of(work, struct _irqfd, shutdown);
+
+ /*
+ * Synchronize with the wait-queue and unhook ourselves to prevent
+ * further events.
+ */
+ remove_wait_queue(irqfd->wqh, &irqfd->wait);
+
+ /*
+ * We know no new events will be scheduled at this point, so block
+ * until all previously outstanding events have completed
+ */
+ flush_work(&irqfd->inject);
+
+ /*
+ * It is now safe to release the object's resources
+ */
+ eventfd_ctx_put(irqfd->eventfd);
+ kfree(irqfd);
+}
+
+/*
+ * Mark the irqfd as inactive and schedule it for removal
+ *
+ * assumes kvm->irqfds.lock is held
+ */
+static void
+irqfd_deactivate(struct _irqfd *irqfd)
+{
+ BUG_ON(!irqfd->active);
+
+ irqfd->active = false;
+ list_del(&irqfd->list);
+
+ queue_work(irqfd_cleanup_wq, &irqfd->shutdown);
+}
+
+/*
+ * Called with wqh->lock held and interrupts disabled
+ */
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;

- /*
- * Assume we will be called with interrupts disabled
- */
if (flags & POLLIN)
- /*
- * Defer the IRQ injection until later since we need to
- * acquire the kvm->lock to do so.
- */
+ /* An event has been signaled, inject an interrupt */
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
- */
- __remove_wait_queue(irqfd->wqh, &irqfd->wait);
- irqfd->wqh = NULL;
+ /* The eventfd is closing, detach from KVM */
+ struct kvm *kvm = irqfd->kvm;
+ unsigned long flags;
+
+ spin_lock_irqsave(&kvm->irqfds.lock, flags);
+
+ if (irqfd->active)
+ /*
+ * If we get here, we can be sure no-one else is
+ * trying to shutdown this object at the same time
+ * since we hold the list lock.
+ */
+ irqfd_deactivate(irqfd);
+
+ spin_unlock_irqrestore(&kvm->irqfds.lock, flags);
}

return 0;
@@ -102,6 +156,7 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
{
struct _irqfd *irqfd;
struct file *file = NULL;
+ struct eventfd_ctx *eventfd = NULL;
int ret;
unsigned int events;

@@ -113,6 +168,8 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
irqfd->gsi = gsi;
INIT_LIST_HEAD(&irqfd->list);
INIT_WORK(&irqfd->inject, irqfd_inject);
+ INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
+ irqfd->active = true;

file = eventfd_fget(fd);
if (IS_ERR(file)) {
@@ -120,6 +177,14 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
goto fail;
}

+ eventfd = eventfd_ctx_fileget(file);
+ if (IS_ERR(file)) {
+ ret = PTR_ERR(file);
+ goto fail;
+ }
+
+ irqfd->eventfd = eventfd;
+
/*
* Install our own custom wake-up handling so we are notified via
* a callback whenever someone signals the underlying eventfd
@@ -129,12 +194,13 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)

events = file->f_op->poll(file, &irqfd->pt);

- mutex_lock(&kvm->lock);
- list_add_tail(&irqfd->list, &kvm->irqfds);
- mutex_unlock(&kvm->lock);
+ spin_lock_irq(&kvm->irqfds.lock);
+ list_add_tail(&irqfd->list, &kvm->irqfds.items);
+ spin_unlock_irq(&kvm->irqfds.lock);

/*
- * Check if there was an event already queued
+ * Check if there was an event already pending on the eventfd
+ * before we registered, and trigger it as if we didn't miss it.
*/
if (events & POLLIN)
schedule_work(&irqfd->inject);
@@ -148,6 +214,9 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
return 0;

fail:
+ if (eventfd && !IS_ERR(eventfd))
+ eventfd_ctx_put(eventfd);
+
if (file && !IS_ERR(file))
fput(file);

@@ -158,24 +227,52 @@ fail:
void
kvm_irqfd_init(struct kvm *kvm)
{
- INIT_LIST_HEAD(&kvm->irqfds);
+ spin_lock_init(&kvm->irqfds.lock);
+ INIT_LIST_HEAD(&kvm->irqfds.items);
}

+/*
+ * This function is called as the kvm VM fd is being released. Shutdown all
+ * irqfds that still remain open
+ */
void
kvm_irqfd_release(struct kvm *kvm)
{
struct _irqfd *irqfd, *tmp;

- list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list) {
- if (irqfd->wqh)
- remove_wait_queue(irqfd->wqh, &irqfd->wait);
+ spin_lock_irq(&kvm->irqfds.lock);

- flush_work(&irqfd->inject);
+ list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds.items, list)
+ irqfd_deactivate(irqfd);

- mutex_lock(&kvm->lock);
- list_del(&irqfd->list);
- mutex_unlock(&kvm->lock);
+ spin_unlock_irq(&kvm->irqfds.lock);

- kfree(irqfd);
- }
+ /*
+ * Block until we know all outstanding shutdown jobs have completed
+ * since we do not take a kvm* reference.
+ */
+ flush_workqueue(irqfd_cleanup_wq);
+
+}
+
+/*
+ * create a host-wide workqueue for issuing deferred shutdown requests
+ * aggregated from all vm* instances. We need our own isolated single-thread
+ * queue to prevent deadlock against flushing the normal work-queue.
+ */
+static int __init irqfd_module_init(void)
+{
+ irqfd_cleanup_wq = create_singlethread_workqueue("kvm-irqfd-cleanup");
+ if (!irqfd_cleanup_wq)
+ return -ENOMEM;
+
+ return 0;
}
+
+static void __exit irqfd_module_exit(void)
+{
+ destroy_workqueue(irqfd_cleanup_wq);
+}
+
+module_init(irqfd_module_init);
+module_exit(irqfd_module_exit);


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