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Date: Tue, 2 Mar 2004 13:21:23 -0500 (EST)
From: "Richard B. Johnson" <root@chaos.analogic.com>
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To: Linux kernel <linux-kernel@vger.kernel.org>
Subject: poll() in 2.6 and beyond
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Poll in 2.6.0; when a driver routine calls poll_wait()
it returns <<immediately>> to somewhere in the
kernel, then waits for my wake_up_interuptible(), before
returning control to a user sleeping in poll(). This means
that the user gets the wrong poll return value! It
doesn't get the value it was given as a result of the
interrupt, but the value that existed (0) before the
interrupt occurred.

Poll should not return from poll_wait() until it gets
a wake_up_interruptible() call. The wait variable,
info->pwait, below, has been initialized by executing
init_waitqueue_head(&info->pwait) in the initialization
code. This code works in 2.4.24.

What do I do to make it work in 2.6.0 and beyond? There
are no hints in the 2.6 drivers as they all seem to be
written like this and they presumably work.


/*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=*/
/*
 *   The interrupt service routine.
 */
static void pci_isr(int irq, void *p, struct pt_regs *regs)
{
    spin_lock(&info->lock);
    DEB(printk(KERN_INFO"%s : Interrupt!\n", devname));
    info->poll_flag |= POLLIN|DEF_POLL;
    wake_up_interruptible(&info->pwait);
    spin_unlock(&info->lock);
}
/*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=*/
/*
 *  Device poll routine.
 */
static size_t poll(struct file *fp, struct poll_table_struct *wait)
{
    size_t poll_flag;
    size_t flags;
    DEB(printk(KERN_INFO"%s : poll called\n", devname));
    poll_wait(fp, &info->pwait, wait);
    lockit(TRUE, &flags);
    poll_flag = info->poll_flag;
    info->poll_flag = 0;
    lockit(FALSE, &flags);
    DEB(printk(KERN_INFO"%s : poll returns\n", devname));
    return poll_flag;
}

Cheers,
Dick Johnson
Penguin : Linux version 2.4.24 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.