From patchwork Mon Jun 29 15:44:15 2009 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Gregory Haskins X-Patchwork-Id: 32971 Received: from vger.kernel.org (vger.kernel.org [209.132.176.167]) by demeter.kernel.org (8.14.2/8.14.2) with ESMTP id n5TFiWni027100 for ; Mon, 29 Jun 2009 15:44:32 GMT Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759450AbZF2PoX (ORCPT ); Mon, 29 Jun 2009 11:44:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758533AbZF2PoX (ORCPT ); Mon, 29 Jun 2009 11:44:23 -0400 Received: from victor.provo.novell.com ([137.65.250.26]:33652 "EHLO victor.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759828AbZF2PoU (ORCPT ); Mon, 29 Jun 2009 11:44:20 -0400 Received: from dev.haskins.net (prv-ext-foundry1.gns.novell.com [137.65.251.240]) by victor.provo.novell.com with ESMTP (TLS encrypted); Mon, 29 Jun 2009 09:44:17 -0600 Received: from dev.haskins.net (localhost [127.0.0.1]) by dev.haskins.net (Postfix) with ESMTP id AF44A4641F6; Mon, 29 Jun 2009 11:44:15 -0400 (EDT) From: Gregory Haskins Subject: [KVM PATCH v6 3/4] KVM: Fix races in irqfd using new eventfd_kref_get interface To: kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, mst@redhat.com, avi@redhat.com, davidel@xmailserver.org Date: Mon, 29 Jun 2009 11:44:15 -0400 Message-ID: <20090629154415.31959.2666.stgit@dev.haskins.net> In-Reply-To: <20090629153627.31959.76947.stgit@dev.haskins.net> References: <20090629153627.31959.76947.stgit@dev.haskins.net> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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 --- include/linux/kvm_host.h | 5 + virt/kvm/Kconfig | 1 virt/kvm/eventfd.c | 154 ++++++++++++++++++++++++++++++++++++++-------- 3 files changed, 131 insertions(+), 29 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html 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 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; }; +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); } + 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);