Subject: Re: poll() in 2.6 and beyond
From: David Dillow <dave@thedillows.org>
To: root@chaos.analogic.com
Cc: Bill Davidsen <davidsen@tmr.com>, Roland Dreier <roland@topspin.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <Pine.LNX.4.53.0403021817050.9351@chaos>
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Date: 02 Mar 2004 22:57:01 -0500
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On Tue, 2004-03-02 at 18:32, Richard B. Johnson wrote:
> Yes. The code I attached earlier shows that the poll() in a driver
> gets called (correctly), then it calls poll_wait(). Unfortunately
> the call to poll_wait() returns immediately so that the return
> value from the driver's poll() is whatever it was before some
> event occurred that the driver was going to signal with
> wake_up_interruptible().

You've been handed a clue enough times now that you should understand
that poll_wait() does not, and has never, put the process to sleep.

If you can show a case where do_poll() returns stale data, then by all
means do so. We will be happy to fix any such error in the kernel.

You say do_poll() looses the status returned from your driver's poll
method. If your driver is truly returning a nonzero status from the
poll() method call, then a simple read of the code in do_pollfd() will
show that the only way it looses information from that event mask is if
your user space is not setting that event type in pollfd.events.

If I were you, I'd check two things:
1) that your poll method is really returning a non-zero status when you
think it is
2) that your user space program is really asking for all events you
think it is

I think you'll find your problem is not this well-used mechanism in the
kernel.

Dave
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