linux-kernel - Re: [patch 1/2] epoll fix own poll()

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Date:	Thu, 29 Jan 2009 23:41:53 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Pavel Pisa <pisa@....felk.cvut.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 1/2] epoll fix own poll()

On Fri, 30 Jan 2009, Pavel Pisa wrote:

> epoll_wait() unconditionally calls  ep_poll(), the "rdllist" is not empty
> (cheked by previous ep_eventpoll_poll() call), so ep_send_events()
> has to be called. It moves all events from "rdllist" onto  "txlist"
> and reports events confirmed by individual poll() calls to user.
> There could be non signaling events in the "rdlist" left there from
> previous scan for level triggering events or in case, that condition
> is reset by some other intervention before  ep_eventpoll_poll().
> All this is not problem yet, it would result in one abundant call to
> epoll_wait() which returns 0. No problem. The EPOLLET events
> are moved from "rdlist", others (level triggered events) signalling
> are queued on "rdlist" again. If userspace does its duty, they would
> be removed during next call. This all seems to suggests, that one
> abundant epoll_wait() would get things back on the right path.
> All critical processing (moving between lists) seems to be under
> held of spinlock, so possibility for bugs is low. I have not yet analyzed
> overflow list which complicates things.
> 
> I am not clever enough, to see, where is the problem, that situation
> is not stabilized. Can you teach me Davide, please?

Sorry, I missed part of the email. Yes, if revents is zero in the 
send-lood, I don't see how files can be added back to the ready-list.
Please, create a very small test program that exhibit such behaviour ...



- Davide


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