linux-kernel - Re: [KVM PATCH v5 3/4] KVM: Fix races in irqfd using new eventfd_kref

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Date:	Sun, 28 Jun 2009 16:18:59 +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,
	paulmck@...ux.vnet.ibm.com, davidel@...ilserver.org,
	rusty@...tcorp.com.au
Subject: Re: [KVM PATCH v5 3/4] KVM: Fix races in irqfd using new
	eventfd_kref_get interface

On Sun, Jun 28, 2009 at 08:50:28AM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Thu, Jun 25, 2009 at 09:28:27AM -0400, Gregory Haskins wrote:
> >   
> >> @@ -65,25 +134,39 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> >>  	unsigned long flags = (unsigned long)key;
> >>  
> >>  	/*
> >> -	 * Assume we will be called with interrupts disabled
> >> +	 * 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
> >> -		 */
> >> +		/* The eventfd is closing, detach from KVM */
> >> +		struct kvm *kvm = irqfd->kvm;
> >> +		unsigned long flags;
> >> +
> >>  		__remove_wait_queue(irqfd->wqh, &irqfd->wait);
> >> -		irqfd->wqh = NULL;
> >> +
> >> +		spin_lock_irqsave(&kvm->irqfds.lock, flags);
> >> +
> >> +		if (irqfd->active) {
> >> +			/*
> >> +			 * If the item is still active we can be sure that
> >> +			 * no-one else is trying to shutdown this object at
> >> +			 * the same time.
> >> +			 *
> >> +			 * Defer the shutdown to a thread so we can flush
> >> +			 * all remaining inject jobs.  We use a slow-work
> >> +			 * item to prevent a deadlock against the work-queue
> >> +			 */
> >> +			irqfd_deactivate(irqfd);
> >> +			slow_work_enqueue(&irqfd->shutdown);
> >>     
> >
> > Greg, in your patch for slow-work module removal, you write:
> >   "Callers must ensure that their module has at least
> >   one reference held while the work is enqueued."
> >   Where does this guarantee come from, in this case?
> >   
> The general guarantee comes from the fact that modules naturally have to
> have a reference to be able to call the enqueue function to begin with,
> or the calling function was already racy.  In this particular case, we
> can guarantee that the kvm vm fd is held while our slow-work is active,
> and all slow work is flushed before it is released.  (I guess I am
> assuming that VFS takes a module reference when an fd is opened, but I
> have not verified that it actually does.  If it doesn't, I suppose KVM
> is already racy w.r.t. unloading, independent of my patches)
> 
> -Greg
> 

that could be the case, as we have, for example:

static struct file_operations kvm_vm_fops = {
        .release        = kvm_vm_release,
        .unlocked_ioctl = kvm_vm_ioctl,
        .compat_ioctl   = kvm_vm_ioctl,
        .mmap           = kvm_vm_mmap,
};

with no owner field.

Avi, shouldn't we initialize the owner field to prevent
kvm module from going away while files are open?

-- 
MST
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