linux-kernel - Re: [RFC PATCH] poll(): add poll_wait_set

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Date:	Sun, 31 Oct 2010 16:02:43 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] poll(): add poll_wait_set_exclusive()

On Thu, 7 Oct 2010, Mathieu Desnoyers wrote:

> > Also, doesn't eventpoll already support exclusive polling? I dunno.
> > Davide might be interested in the discussion regardless.
> 
> Looking at epoll(7), the behavior of EPOLLONESHOT when there are multiple epoll
> instances monitoring a file descriptor seems unclear: does it stop event
> propagation after delivery to the first epoll instance (this is the behavior I
> am looking for), or does it stop the event delivery after having woken up all
> epoll instances monitoring the file descriptor ? Davide might have the answer to
> this one.

Sorry for the late response, but I am very slowly wroking my way through a 
long backlog.
In your case above, both fds will get event report, in epoll, with 
EPOLLONESHOT (because the one-shot applies to the epoll-fd/monitored-fd 
pair/key).
As for the exclusive wakeup feature, I am not totally against it, though 
it is borderline as far as use cases vs. added complexity.
The POLLEX/EPOLLEX would be nicer, if it wouldn't lead to a racy interface 
(say one thread uses [E]POLLEX and the another not).
IMO this is more fcntl(2) flag territory, as the application would set the 
behavior globally (for the file), though again, I am not sure the use 
cases justify the introduction and handling of the new flag.



- Davide


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