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
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