Re: [PATCH v2] epoll: Support for disabling items, and a self-testapp.

[Andy Lutomirski <luto at amacapital dot net>]

[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@gmail.com> wrote:
> 
>> On Thu, Aug 23, 2012 at 11:15 PM, Paton J. Lewis <palewis@adobe.com> wrote:
>>> From: "Paton J. Lewis" <palewis@adobe.com>
>>>
>>> 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