Re: [KVM PATCH v10] kvm: add support for irqfd

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Gregory Haskins <ghaskins@novell.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	avi@redhat.com, davidel@xmailserver.org, mtosatti@redhat.com
Subject: Re: [KVM PATCH v10] kvm: add support for irqfd
Date: Thu, 11 Jun 2009 16:16:47 +0300	[thread overview]
Message-ID: <20090611131647.GA15743@redhat.com> (raw)
In-Reply-To: <20090520142234.22285.72274.stgit@dev.haskins.net>

Going over this code again, I seem to see a minor error handling issue here:

On Wed, May 20, 2009 at 10:30:49AM -0400, Gregory Haskins wrote:
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> new file mode 100644
> index 0000000..72a282e
> --- /dev/null
> +++ b/virt/kvm/eventfd.c
> @@ -0,0 +1,228 @@
> +/*
> + * kvm eventfd support - use eventfd objects to signal various KVM events
> + *
> + * Copyright 2009 Novell.  All Rights Reserved.
> + *
> + * Author:
> + *	Gregory Haskins <ghaskins@novell.com>
> + *
> + * This file is free software; you can redistribute it and/or modify
> + * it under the terms of version 2 of the GNU General Public License
> + * as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software Foundation,
> + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
> + */
> +
> +#include <linux/kvm_host.h>
> +#include <linux/workqueue.h>
> +#include <linux/syscalls.h>
> +#include <linux/wait.h>
> +#include <linux/poll.h>
> +#include <linux/file.h>
> +#include <linux/list.h>
> +
> +/*
> + * --------------------------------------------------------------------
> + * irqfd: Allows an fd to be used to inject an interrupt to the guest
> + *
> + * Credit goes to Avi Kivity for the original idea.
> + * --------------------------------------------------------------------
> + */
> +struct _irqfd {
> +	struct kvm               *kvm;
> +	int                       gsi;
> +	struct file              *file;
> +	struct list_head          list;
> +	poll_table                pt;
> +	wait_queue_head_t        *wqh;
> +	wait_queue_t              wait;
> +	struct work_struct        work;
> +};
> +
> +static void
> +irqfd_inject(struct work_struct *work)
> +{
> +	struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
> +	struct kvm *kvm = irqfd->kvm;
> +
> +	mutex_lock(&kvm->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->lock);
> +}
> +
> +static int
> +irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> +	struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
> +
> +	/*
> +	 * The 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.
> +	 */
> +	schedule_work(&irqfd->work);
> +
> +	return 0;
> +}
> +
> +static void
> +irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
> +			poll_table *pt)
> +{
> +	struct _irqfd *irqfd = container_of(pt, struct _irqfd, pt);
> +
> +	irqfd->wqh = wqh;
> +	add_wait_queue(wqh, &irqfd->wait);
> +}
> +
> +static int
> +kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
> +{
> +	struct _irqfd *irqfd;
> +	struct file *file = NULL;
> +	int ret;
> +
> +	irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL);
> +	if (!irqfd)
> +		return -ENOMEM;
> +
> +	irqfd->kvm = kvm;
> +	irqfd->gsi = gsi;
> +	INIT_LIST_HEAD(&irqfd->list);
> +	INIT_WORK(&irqfd->work, irqfd_inject);
> +
> +	/*
> +	 * Embed the file* lifetime in the irqfd.
> +	 */
> +	file = fget(fd);
> +	if (IS_ERR(file)) {
> +		ret = PTR_ERR(file);
> +		goto fail;
> +	}
> +
> +	/*
> +	 * Install our own custom wake-up handling so we are notified via
> +	 * a callback whenever someone signals the underlying eventfd
> +	 */
> +	init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
> +	init_poll_funcptr(&irqfd->pt, irqfd_ptable_queue_proc);
> +
> +	ret = file->f_op->poll(file, &irqfd->pt);
> +	if (ret < 0)
> +		goto fail;
> +
> +	irqfd->file = file;
> +
> +	mutex_lock(&kvm->lock);
> +	list_add_tail(&irqfd->list, &kvm->irqfds);
> +	mutex_unlock(&kvm->lock);
> +
> +	return 0;
> +
> +fail:
> +	if (irqfd->wqh)
> +		remove_wait_queue(irqfd->wqh, &irqfd->wait);

Why are these 2 lines here? Either we might get a callback even though
poll failed - and then this test without lock is probably racy -
or we can't, and then we can replace the above with BUG_ON(irqfd->wqh).

Which is it? I think the later ...

> +
> +	if (file && !IS_ERR(file))
> +		fput(file);
> +
> +	kfree(irqfd);
> +	return ret;
> +}
> +
> +static void
> +irqfd_release(struct _irqfd *irqfd)
> +{
> +	/*
> +	 * The ordering is important.  We must remove ourselves from the wqh
> +	 * first to ensure no more event callbacks are issued, and then flush
> +	 * any previously scheduled work prior to freeing the memory
> +	 */
> +	remove_wait_queue(irqfd->wqh, &irqfd->wait);
> +
> +	flush_work(&irqfd->work);
> +
> +	fput(irqfd->file);
> +	kfree(irqfd);
> +}
> +
> +static struct _irqfd *
> +irqfd_remove(struct kvm *kvm, struct file *file, int gsi)
> +{
> +	struct _irqfd *irqfd;
> +
> +	mutex_lock(&kvm->lock);
> +
> +	/*
> +	 * linear search isn't brilliant, but this should be an infrequent
> +	 * slow-path operation, and the list should not grow very large
> +	 */
> +	list_for_each_entry(irqfd, &kvm->irqfds, list) {
> +		if (irqfd->file != file || irqfd->gsi != gsi)
> +			continue;
> +
> +		list_del(&irqfd->list);
> +		mutex_unlock(&kvm->lock);
> +
> +		return irqfd;
> +	}
> +
> +	mutex_unlock(&kvm->lock);
> +
> +	return NULL;
> +}
> +
> +static int
> +kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
> +{
> +	struct _irqfd *irqfd;
> +	struct file *file;
> +	int count = 0;
> +
> +	file = fget(fd);
> +	if (IS_ERR(file))
> +		return PTR_ERR(file);
> +
> +	while ((irqfd = irqfd_remove(kvm, file, gsi))) {
> +		/*
> +		 * We remove the item from the list under the lock, but we
> +		 * free it outside the lock to avoid deadlocking with the
> +		 * flush_work and the work_item taking the lock
> +		 */
> +		irqfd_release(irqfd);
> +		count++;
> +	}
> +
> +	fput(file);
> +
> +	return count ? count : -ENOENT;
> +}
> +
> +int
> +kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> +{
> +	if (flags & KVM_IRQFD_FLAG_DEASSIGN)
> +		return kvm_deassign_irqfd(kvm, fd, gsi);
> +
> +	return kvm_assign_irqfd(kvm, fd, gsi);
> +}
> +
> +void
> +kvm_irqfd_release(struct kvm *kvm)
> +{
> +	struct _irqfd *irqfd, *tmp;
> +
> +	/* don't bother with the lock..we are shutting down */
> +	list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list) {
> +		list_del(&irqfd->list);
> +		irqfd_release(irqfd);
> +	}
> +}

-- 
MST