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. It also
converts the eventfd POLLHUP generation code to use the locked variant
of wakeup.
Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
CC: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
---
fs/eventfd.c | 7 --
include/linux/kvm_host.h | 5 +
virt/kvm/eventfd.c | 187 ++++++++++++++++++++++++++++++++--------------
3 files changed, 134 insertions(+), 65 deletions(-)
diff --git a/fs/eventfd.c b/fs/eventfd.c
index d9849a1..31d12de 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -105,12 +105,7 @@ static int eventfd_release(struct inode *inode, struct file *file)
{
struct eventfd_ctx *ctx = file->private_data;
- /*
- * No need to hold the lock here, since we are on the file cleanup
- * path and the ones still attached to the wait queue will be
- * serialized by wake_up_locked_poll().
- */
- wake_up_locked_poll(&ctx->wqh, POLLHUP);
+ wake_up_poll(&ctx->wqh, POLLHUP);
eventfd_ctx_put(ctx);
return 0;
}
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1a8952f..7605bc4 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 a9e7de7..05ce447 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -28,7 +28,6 @@
#include <linux/file.h>
#include <linux/list.h>
#include <linux/eventfd.h>
-#include <linux/srcu.h>
/*
* --------------------------------------------------------------------
@@ -37,66 +36,86 @@
* Credit goes to Avi Kivity for the original idea.
* --------------------------------------------------------------------
*/
+
struct _irqfd {
- struct mutex lock;
- struct srcu_struct srcu;
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;
};
+static struct workqueue_struct *irqfd_cleanup_wq;
+
static void
irqfd_inject(struct work_struct *work)
{
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->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);
- }
+ struct kvm *kvm = irqfd->kvm;
- srcu_read_unlock(&irqfd->srcu, idx);
+ 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);
}
+/*
+ * Race-free decouple logic (ordering is critical)
+ */
static void
-irqfd_disconnect(struct _irqfd *irqfd)
+irqfd_shutdown(struct work_struct *work)
{
- struct kvm *kvm;
+ struct _irqfd *irqfd = container_of(work, struct _irqfd, shutdown);
- mutex_lock(&irqfd->lock);
+ /*
+ * 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);
+}
- kvm = rcu_dereference(irqfd->kvm);
- rcu_assign_pointer(irqfd->kvm, NULL);
- mutex_unlock(&irqfd->lock);
+/* assumes kvm->irqfds.lock is held */
+static bool
+irqfd_is_active(struct _irqfd *irqfd)
+{
+ return list_empty(&irqfd->list) ? false : true;
+}
- if (!kvm)
- return;
+/*
+ * 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_is_active(irqfd));
- mutex_lock(&kvm->lock);
- list_del(&irqfd->list);
- mutex_unlock(&kvm->lock);
+ list_del_init(&irqfd->list);
- /*
- * 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);
+ 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)
{
@@ -104,25 +123,29 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
unsigned long flags = (unsigned long)key;
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.
- */
+ /* An event has been signaled, inject an interrupt */
schedule_work(&irqfd->inject);
if (flags & POLLHUP) {
+ /* The eventfd is closing, detach from KVM */
+ struct kvm *kvm = irqfd->kvm;
+ unsigned long flags;
+
+ spin_lock_irqsave(&kvm->irqfds.lock, flags);
+
/*
- * 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()
+ * We must check if someone deactivated the irqfd before
+ * we could acquire the irqfds.lock since the item is
+ * deactivated from the KVM side before it is unhooked from
+ * the wait-queue. If it is already deactivated, we can
+ * simply return knowing the other side will cleanup for us.
+ * We cannot race against the irqfd going away since the
+ * other side is required to acquire wqh->lock, which we hold
*/
- remove_wait_queue(irqfd->wqh, &irqfd->wait);
- flush_work(&irqfd->inject);
- irqfd_disconnect(irqfd);
+ if (irqfd_is_active(irqfd))
+ irqfd_deactivate(irqfd);
- cleanup_srcu_struct(&irqfd->srcu);
- kfree(irqfd);
+ spin_unlock_irqrestore(&kvm->irqfds.lock, flags);
}
return 0;
@@ -143,6 +166,7 @@ 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;
@@ -150,12 +174,11 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
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->inject, irqfd_inject);
+ INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
file = eventfd_fget(fd);
if (IS_ERR(file)) {
@@ -163,6 +186,14 @@ 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
@@ -172,14 +203,13 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
events = file->f_op->poll(file, &irqfd->pt);
- kvm_get_kvm(kvm);
-
- 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);
@@ -193,6 +223,9 @@ 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);
@@ -203,14 +236,52 @@ 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)
- irqfd_disconnect(irqfd);
+ spin_lock_irq(&kvm->irqfds.lock);
+
+ list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds.items, list)
+ irqfd_deactivate(irqfd);
+
+ spin_unlock_irq(&kvm->irqfds.lock);
+
+ /*
+ * 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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