At 8/14/2012 01:21 PM, Christof Meerwald wrote:
Hi Paton, On Thu, Aug 02, 2012 at 06:37:06PM -0700, Paton J. Lewis wrote: [...] > My first concern is about code clarity. Using a custom event to > delete an event type (either EPOLLIN or EPOLLOUT) from an epoll item > requires that functionality to be split across two areas of code: > the code that requests the deletion (via the call to epoll_ctl), and > the code that responds to it (via epoll_wait). But don't you have a similar problem in your proposal as well as you might get an EBUSY when trying to disabling the item - in which case you would have to do the deletion in the epoll_wait loop.
Good point.
> However, my main concern is about performance. Handling a custom > event means that each return from epoll_wait requires the responding > thread to check for possible custom events, which in the case of > deletion is going to be relatively rare. Thus code which was once > purely concerned with responding to I/O events must now spend a > fraction of its time testing for exceptional conditions. In > addition, handling deletion in this manner now requires a thread or > context switch. But in your initial proposal you also had the code checking for deletion in the epoll_wait loop.
Also true. However, I believe the context switch is always required by the custom message passing technique, but will not always happen when using the event disabling technique.
> Given the drawbacks listed above, and the kernel design philosophy > of only implementing what is actually needed, I would argue for > sticking with the original EPOLL_CTL_DISABLE proposal for now. I have finally had some chance to play around with your patch a bit and I really think that you don't want to check for ep_is_linked(&epi->rdllink) in ep_disable as I don't see that this would provide any useful semantics with respect to race-conditions. I.e. consider the point in the epoll_wait loop just after you have re-enabled to item - in this case ep_disable would (almost certainly) return EBUSY, but there is no guarantee that epoll_wait will be woken up on the next iteration. As I mentioned, I think it would be much more useful to check for "epi->event.events & ~EP_PRIVATE_BITS" instead which I believe would provide more useful semantics.
You are correct. Thanks for being patient and persistent here. I discovered this problem myself last week during testing, and I am planning to post a v2 patch proposal that includes this fix.
I am also working on an epoll self-test as Andrew Morton suggested. I'm going to finish that before re-submitting the EPOLL_CTL_DISABLE patch to help reduce the possibility that the v2 patch still contains bugs.
Pat
Christof -- http://cmeerw.org sip:cmeerw at cmeerw.org mailto:cmeerw at cmeerw.org xmpp:cmeerw at cmeerw.org
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