Michael S. Tsirkin wrote: > 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? > Yep, creating your own queue works too. I picked slow-work as an alternate to generating a dedicated resource, but I agree either method would work fine. Do you have a preference? Regards, -Greg > > >
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