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@xxxxxxxxxx> > --- > > 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? > 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. 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. > }; > > +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 > 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); -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html