Re: [KVM PATCH v5 3/4] KVM: Fix races in irqfd using new eventfd_kref_get interface

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Jun 28, 2009 at 08:53:22AM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Thu, Jun 25, 2009 at 09:28:27AM -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.
> >>     
> >
> > By the way, why are we using slow-work here? Wouldn't a regular
> > workqueue do just as well, with less code, and avoid the race?
> >
> >   
> I believe it will cause a problem if you do a "flush_work()" from inside
> a work-item.  I could be wrong, of course, but it looks like a recipe to
> deadlock.
> 
> -Greg
> 

Sure, but the idea is to only flush on kvm close, never from work item.

-- 
MST