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

[Gregory Haskins <ghaskins@novell.com>]

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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


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