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

From: Andy Lutomirski
Date: Thu Oct 18 2012 - 14:05:09 EST

[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
>> []. 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;
rcu_call(delete pdata);

wait() {
for each event pdata {
if (pdata->gone) continue;
process the event;


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

Am I right?

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