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

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Message-ID: <CAMP5XgcN+qBJ7FryzXJEG11F=3ajHDoBncMyShZOksd14SaptA@mail.gmail.com>
Date:	Fri, 22 Mar 2013 15:18:06 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Eric Wong <normalperson@...t.net>
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

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.

-- 
Arve Hjønnevåg
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