Date:	Mon, 29 Jun 2009 20:32:18 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org
Subject: Re: [KVM PATCH v6 3/4] KVM: Fix races in irqfd using new
	eventfd_kref_get interface

On Mon, Jun 29, 2009 at 12:52:24PM -0400, Gregory Haskins wrote:
> 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@...ell.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".

ok.

> >   
> >>  };
> >>  
> >> +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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