Re: poll() in 2.6 and beyond

[Richard B. Johnson]

On Wed, 3 Mar 2004, Linus Torvalds wrote:

>
>
> On Wed, 3 Mar 2004, Linus Torvalds wrote:
> >
> > You should clear the "events pending" flag only when you literally remove
> > the event (ie at "read()" time, not at "poll()" time). Because the
> > select() code _will_ call down to the "poll()" functions multiple times if
> > it gets woken up for any bogus reason.
>
> Hmm.. The above is all still true and your poll() implementation is bad,
> but looking at your test program, the problematic case really shouldn't
> trigger (we should call poll()  multiple times only when it returns zero).
>
> To trigger that bug, you'd have to occasionally call poll() with the
> POLLIN bit clear in the incoming events, which your test program doesn't
> ever do.
>

Well, when I removed the local poll_flag, using only the global one,
and cleared it to zero after the long long was fetched under the lock
(in the read routine), my observed problems go away.

static size_t poll(struct file *fp, struct poll_table_struct *wait)
{
    DEB(printk(KERN_INFO"%s : poll() called\n", devname));
    poll_wait(fp, &pwait, wait);
    DEB(printk(KERN_INFO"%s : poll() returned\n", devname));
    return global_poll_flag;
}

static int read(struct file *fp, char *buf, size_t count, loff_t *ppos)
{
    long long tmp;
    size_t flags;
    spin_lock_irqsave(&rtc_lock, flags);
    tmp = rtc_tick;
    global_poll_flag = 0;
    spin_unlock_irqrestore(&rtc_lock, flags);
    if(copy_to_user(buf, &tmp, sizeof(tmp)))
        return -EFAULT;
    return sizeof(tmp);
}

> Anyway, you should move the clearing to read(), but there may well be
> something else going on too.
>
> What's the frequency you are programming the thing to send interrupts at?
>

2048 ticks/second, trivial to handle.

> 		Linus
> -


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


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