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SubjectRe: [KVM PATCH v6 3/4] KVM: Fix races in irqfd using new eventfd_kref_get interface
Michael S. Tsirkin wrote:
> Look good. A couple of minor nits:
>
> On Mon, Jun 29, 2009 at 11:44:15AM -0400, Gregory Haskins wrote:
>
>> 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.
>>
>> Note that one final race is known to exist: the slow-work thread may race
>> with module removal. We are currently working with slow-work upstream
>> to fix this issue as well. Since the code prior to this patch also
>> races with module_put(), we are not making anything worse, but rather
>> shifting the cause of the race. Once the slow-work code is patched we
>> will be fixing the last remaining issue.
>>
>> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
>> ---
>>
>> include/linux/kvm_host.h | 5 +
>> virt/kvm/Kconfig | 1
>> virt/kvm/eventfd.c | 154 ++++++++++++++++++++++++++++++++++++++--------
>> 3 files changed, 131 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/Kconfig b/virt/kvm/Kconfig
>> index daece36..ab7848a 100644
>> --- a/virt/kvm/Kconfig
>> +++ b/virt/kvm/Kconfig
>> @@ -9,6 +9,7 @@ config HAVE_KVM_IRQCHIP
>> config HAVE_KVM_EVENTFD
>> bool
>> select EVENTFD
>> + select SLOW_WORK
>>
>
> not needed in this version?
>

Good catch. Will remove.

>
>> config KVM_APIC_ARCHITECTURE
>> bool
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> index 9656027..76ad125 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;
>>
>
> I think we need a comment here that active bit is protected by irqfds
> lock.

Ack

> One idea I had to make it even clearer was to have a shutdown list
> of irqfds per-kvm, together with the items list, and make work_struct for
> shutdown global, not per-irqfd. We can then unconditionally do
> list_move + schedule_work to shut down an irqfd, and it's safe to do
> even if it is already on the shutdown list - it just gets moved to tail.
>

Hmm..I'm not sure that churn really buys us anything, tho. Technically
the "active" bit is redundant with list_del_init()+list_empty() that I
employed in previous versions. However, I made it explicit with the
active bit to be more self-documenting. IMO, the latest code is pretty
clear, and the change you are proposing is moving towards a slightly
trickier variant like I originally had. I'd say "lets leave this as is".

>
>> };
>>
>> +static struct workqueue_struct *irqfd_cleanup_wq;
>> +
>> static void
>> irqfd_inject(struct work_struct *work)
>> {
>> @@ -58,32 +64,81 @@ 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);
>> }
>>
>> +
>>
>
> extra empty line
>

Ack
>
>> return 0;
>> }
>>
>> @@ -102,6 +157,7 @@ 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 +169,8 @@ 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 +178,14 @@ 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 +195,13 @@ 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 +215,9 @@ 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 +228,52 @@ 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);
>> +
>> + /*
>> + * Block until we know all outstanding shutdown jobs have completed
>> + * since we do not take a kvm* reference.
>> + */
>> + flush_workqueue(irqfd_cleanup_wq);
>>
>> - kfree(irqfd);
>> - }
>> }
>> +
>> +/*
>> + * 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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