linux-kernel - Re: [RFC v3 1/2] epoll: avoid spinlock contention with wfcqueue

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Date:	Fri, 22 Mar 2013 22:54:28 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC v3 1/2] epoll: avoid spinlock contention with wfcqueue

Arve Hjønnevåg <arve@...roid.com> wrote:
> On Fri, Mar 22, 2013 at 3:31 AM, Eric Wong <normalperson@...t.net> wrote:
> > Arve Hjønnevåg <arve@...roid.com> wrote:
> >> On Thu, Mar 21, 2013 at 8:24 PM, Eric Wong <normalperson@...t.net> wrote:
> >> >
> >> > With EPOLLET and improper usage (not hitting EAGAIN), the event now
> >> > has a larger window to be lost (as mentioned in my changelog).
> >> >
> >>
> >> What about the case where EPOLLET is not set? The old code did not
> >> drop events in that case.
> >
> > Nothing is dropped, if the event wasn't on the ready list before,
> > ep_poll_callback may still append the ready list while __put_user
> > is running.
> >
> > If the event was on the ready list:
> >
> > 1) It does not matter for EPOLLONESHOT, it'll get masked out and
> >    discarded in the next ep_send_events call until ep_modify reenables
> >    it.  Since ep_modify and ep_send_events both take ep->mtx, there's
> >    no conflict.
> >
> > 2) Level Trigger - event stays ready, so nothing is dropped.
> >
> 
> At some point the level triggered event has to get cleared. As far as
> I can tell, your new code will drop new events that occur between
> "revents = ep_item_poll(epi, &pt);" and "epi->state = EP_STATE_IDLE;"
> in that case.

Thanks for catching that, I'll need to fix that.  Maybe reintroduce
EP_STATE_DEQUEUE, but just for the (LT && !revents) case.
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