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To: root@chaos.analogic.com
Cc: Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: poll() in 2.6 and beyond
References: <Pine.LNX.4.53.0403021318580.796@chaos>
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From: Roland Dreier <roland@topspin.com>
Date: 02 Mar 2004 12:04:43 -0800
In-Reply-To: <Pine.LNX.4.53.0403021318580.796@chaos>
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    Richard> Poll in 2.6.0; when a driver routine calls poll_wait() it
    Richard> returns <<immediately>> to somewhere in the kernel, then
    Richard> waits for my wake_up_interuptible(), before returning
    Richard> control to a user sleeping in poll(). This means that the
    Richard> user gets the wrong poll return value! It doesn't get the
    Richard> value it was given as a result of the interrupt, but the
    Richard> value that existed (0) before the interrupt occurred.

Nothing has changed in 2.6 that I know of.  poll_wait() always
returned immediately to the driver.  The driver's poll method is
supposed to call poll_wait() on the wait queues that indicate a change
in poll status, and then return with the current status.

Read the description of "poll and select" in LDD:
    <http://www.xml.com/ldd/chapter/book/ch05.html#t3>

 - Roland

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