Re: [Xenomai-core] Enhanced RTDM device closure

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Hi,
>>
>> a few changes of the RTDM layer were committed to trunk recently. They
>> make handling of RTDM file descriptors more handy:
>>
>>  o rt_dev_close/POSIX-close now polls as long as the underlying device
>>    reports -EAGAIN. No more looping inside the application is required.
>>    This applies to the usual non-RT invocation of close, the corner
>>    case "close from RT context" can still return EAGAIN.
>>
>>  o Automatic cleanup of open file descriptors has been implemented. This
>>    is not yet the perfect design (*), but a straightforward approach to
>>    ease the cleanup after application crashes or other unexpected
>>    terminations.
>>
>> The code is still young, so testers are welcome.
>>
>> Jan
>>
>>
>> (*) Actually, I would like to see generic per-process file descriptor
>> tables one day, used by both the POSIX and the RTDM skin. The FD table
>> should be obtained via xnshadow_ppd_get().
> 
> I agree for the file descriptor table, but I do not see why it should be
> bound to xnshadow_ppd_get. The file descriptor table could be
> implemented in an object like fashion, where the caller is responsible
> to pass the same pointer to the creation, use and desctruction routines.

But where to get this pointer from when I enter, say, rtdm_ioctl on
behalf of some process? The caller just passes an integer, the file
descriptor.

> This would allow, for example, to have a descriptor table for
> kernel-space threads. Another feature that would be interesting for the

I don't see the need to offer kernel threads private fd tables. They can
perfectly continue to use a common, then kernel-only table. There are
too few of those threads, and there is no clear concept of a process
boundary in kernel space.

> posix skin would be to have a callback called at process fork time in
> order to duplicate the fd table.

Ack. IIRC, this callback could also serve to solve the only consistency
issue of the ipipe_get_ptd() approach.

> 
> 
>  But first this requires
>> lock-less xnshadow_ppd_get() based on ipipe_get_ptd() to keep the
>> overhead limited. Yet another story.
> 
> xnshadow_ppd_get is already lockless, usual callers have to hold the
> nklock for other reasons anyway.
> 

OK, depends on the POV :). Mine is that the related RTDM services do not
hold nklock and will never have to. Moreover, there is no need for
locking design-wise, because per-process data cannot vanish under the
caller unless the caller vanishes. The need currently only comes from
the hashing-based lookup (reminds me of the WCET issues kernel futexes
have...).

Jan


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