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 12:28:19PM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Sun, Jun 28, 2009 at 03:57:30PM +0300, Michael S. Tsirkin wrote:
> >   
> >> On Sun, Jun 28, 2009 at 03:56:12PM +0300, Michael S. Tsirkin wrote:
> >>     
> >>> 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.
> >>>       
> >> To clarify, you don't flush slow works from a work-item,
> >> so you shouldn't need to flush workqueue either.
> >>     
> >
> > I guess my question is - why is slow work different? It's still
> > a thread pool underneath ...
> >
> >   
> Its not interdependent.  Flush-work blocks the thread..if the thread
> happens to be the work-queue thread you may deadlock preventing it from
> processing further jobs like the inject.  In reality it shouldnt be
> possible, but its just a bad idea to assume its ok.
> Slow work, on the
> other hand, will just make a new thread.
> 
> -Greg
> 

But if you create your own workqueue, and all you do there is destroy
irqfds, things are ok I think. Right?