Re: poll() in 2.6 and beyond

["Richard B. Johnson" <root@chaos.analogic.com>]

On Wed, 3 Mar 2004, Linus Torvalds wrote:

>
>
> On Wed, 3 Mar 2004, Richard B. Johnson wrote:
> >
> > are being quashed by those who just like to argue. Therefore,
> > I wrote some code that emulates the environment in which I
> > discovered the poll failure.
>
> No. I think you wrote some code that shows the bug you have.
>
> Your "poll()" function IS BUGGY.
>
> Look at this:
>
> 	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, &pwait, wait);
> 	    DEB(printk(KERN_INFO"%s : poll() returned\n", devname));
> 	    spin_lock_irqsave(&rtc_lock, flags);
> 	    poll_flag = global_poll_flag;
> ***	    global_poll_flag = 0;
> 	    spin_unlock_irqrestore(&rtc_lock, flags);
> 	    return poll_flag;
> 	}
>
> you are clearing your own flag that says "there are events pending", so if
> you call your "poll()" function twice, on the second time it will say
> "there are no events pending".
>

But I am clearing it under a spin-lock.

> 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.

Oh wow. That is the BUG! I didn't know it could call in multiple times.

>
> See if that fixes anything.
>
> It may well be that 2.6.x calls down to the low-level driver "poll()"
> function more than it should. That would be a mis-feature, and worth
> looking at, but I think you should try to fix your test first, since right
> now the bug is questionable.
>
> 			Linus

And YES! If I clear the flag only after it is read. It "fixes" the
observed problem!

I don't know if it is a BUG or a FEATURE, but you put your
finger right on it!  Thanks.


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