Re: Behavior of poll() within a module

[Manfred Spraul]

From: "Richard B. Johnson" <[e-mail]>
> 
> In this module, there isn't any read() or write() event that can
> clear the poll mask. Instead, the sole purpose of poll is to tell
> the user-mode caller that there is new status available as a result
> of an interrupt. This is a module that controls a motor. It runs
> <forever> on its own. It gets new parameters via an ioctl(). It
> reports exceptions (like overload conditions) using the poll
> mechanism.
> 
> The caller reads the cached status via an ioctl(). Any caller sleeping
> in poll must be awakened as a result of the interrupt event. Any caller
> can read the cached status at any time. If this was allowed to
> clear the poll mask, only one caller would be awakened. 
>
Ugh.
->poll must never change any state. The kernel is free to call poll
multiple times (common are once, twice and three times).

Can you use a per-filp pollmask?
* remove poll_active
* remove poll_mask
* add event counters for every possible event.
    poll_count_POLLIN, poll_count_POLLOUT, etc.
* interrupt handler increases the counter.
* ioctl() copies the value of the counters into
    filp->private_data->event_handled_POLL{IN,OUT}
* poll sets the pollmask if
    filp->private_data->event_seen != info->poll_count

If filps (file descriptors) are shared between apps, then I have no
idea how to fix your design.

--
    Manfred