Re: poll() in 2.6 and beyond

[Richard B. Johnson]

On Tue, 2 Mar 2004, John Muir wrote:

> Richard B. Johnson wrote:
>
> > > Could you maybe go back to the initial report, which is that after
> > > poll() gets wrong status? It's nice to argue about where the process
> > > waits, but the issue is if it gets the same status with 2.4 and 2.6,
> > and
> > > if not which one should be fixed.
> > >
> > > Richard: can you show this with a small demo program? I assume you
> > > didn't find this just by reading code ;-)
> >
> > 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().
> >
> The documentation referred to earlier
> (http://www.xml.com/ldd/chapter/book/ch05.html#t3) clearly states that
> the poll function implementation for a device should:
> "1. Call /poll_wait/ on one or more wait queues that could indicate a
> change in the poll status.
> 2. Return a bit mask describing operations that could be immediately
> performed without blocking."
> What happens is if your device has data ready RIGHT NOW (without any
> waiting), the information regarding that is returned.
>
> Now, if you look closely at the implementation of do_poll(), it will
> loop forever until any device returns that data is available (or the
> timeout occurs). After data is available, do_pollfd() function no longer
> adds the devices to the wait queue.
>
> What this means is that the first time through each device is added to
> the wait queue. After the process is woken up from schedule_timeout (or
> the timeout occurs), then it will loop through each device again to
> either add that device to the wait queue again, or determine that there
> is data ready to be read/written, or an error or whatever. do_pollfd()
> then increases count, and do_poll() exits from it's loop (and wait
> queues are cleaned-up in sys_poll()).
>
> So, yes, the poll_wait() returns immediately, it should not wait, and
> your poll method should return the device's CURRENT status in the poll
> mask. Don't worry, because when your device wakes the waiting process
> when data is ready, the device's poll function is called again and you
> again can return the CURRENT status.
>

Well the device's poll function isn't getting called the second time
with 2.6.0. I never checked it in 2.4.x because it always worked.
This problem occurs in a driver that only returns the fact that
one event occurred. When it failed to report the event when built
with a newer kernel, I added diagnostics which showed that the
poll in the driver was only called once --and that the return from
poll_wait happened immediately.

So, if the poll_wait isn't a wait-function, but just some add-wakeup
to the queue function, then its name probably should have been
changed when it changed. At one time it did, truly, wait until
it was awakened with wake_up_interruptible.

> Please let me know if I'm misunderstanding this, but quite frankly,
> poll_wait CANNOT wait, for the very reasons described in previous posts:
> when multiple devices are polled, then poll_wait is called FOR EACH DEVICE.
>
> Cheers,
>
> ..John
>
>

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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