[cc Paul McKenney, who is probably the leading expert on these things] On 10/17/2012 04:30 PM, Andrew Morton wrote: > On Tue, 16 Oct 2012 17:12:57 +0200 > "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> wrote: > >> On Thu, Aug 23, 2012 at 11:15 PM, Paton J. Lewis <palewis@xxxxxxxxx> wrote: >>> From: "Paton J. Lewis" <palewis@xxxxxxxxx> >>> >>> Enhanced epoll_ctl to support EPOLL_CTL_DISABLE, which disables an epoll item. >>> If epoll_ctl doesn't return -EBUSY in this case, it is then safe to delete the >>> epoll item in a multi-threaded environment. Also added a new test_epoll self- >>> test app to both demonstrate the need for this feature and test it. >> >> (There's a lot of background missing from this version of the patch >> that was included in the previous version >> [http://thread.gmane.org/gmane.linux.kernel/1311457]. It helps to >> include the full rationale with a revised patch--best not to assume >> that someone has the context of past mails when reading a revised >> patch.) >> >> I've taken a look at this patch as it currently stands in 3.7-rc1, and >> done a bit of testing. (By the way, the test program >> tools/testing/selftests/epoll/test_epoll.c does not compile...) > > Thanks for this. You raise significant issues. If we can't get these > fully resolved over the next month or so, we should revert the patch so > this new API doesn't get released in 3.7. I have queued a patch to do > this and shall maintain it while I watch developments... I can't shake the feeling that EPOLL_CTL_DISABLE is solving a non-problem, or, more precisely, that there should be a perfectly good userspace solution with no kernel changes. Unless something is rather buggy in kernel land (and I don't think it is), once EPOLL_CTL_DEL has returned, no call to epoll_wait that starts *after* EPOLL_CTL_DEL finishes will return that object. This suggests an RCU-like approach: once EPOLL_CTL_DEL has returned and every thread has returned from an epoll_wait call that started after the EPOLL_CTL_DEL returns, then the data structure can be safely freed. In pseudocode: delete(fd, pdata) { pdata->dead = true; EPOLL_CTL_DEL(fd); rcu_call(delete pdata); } wait() { epoll_wait; for each event pdata { if (pdata->gone) continue; process the event; } rcu_this_is_a_grace_period(); } Of course, these are not normal grace periods and would need to be tracked separately. (The optimal data structure to do this without killing scalability is not obvious. urcu presumably implements such a thing.) Am I right? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
