Re: [Xenomai-core] Context-sharing semaphores

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

Remco den Breeje wrote:
> Hi all,
> 
> I'm having trouble trying to share a semaphore that is created in
> user-space context with a rtdm-driver that runs in kernel-space. I was
> hoping you could explain me what I'm doing wrong.

The design. :)

> 
> I'm using a posix-skin based user-space Xenomai application that runs
> a loop which is triggered by a semaphore.
> This semaphore is posted by a pthread that runs at a specific
> frequency. However, I want to be able to synchronize this loop with an
> external hardware trigger that is read out by a rtdm-driver. To do so,
> I try to disable the semaphore-post in the user-mode pthread, and
> re-use the existing semaphore in kernel-mode (the rtdm-driver).
> Unfortunately, the rtdm-driver is not able to find the user-space
> created semaphore.
> 
> I wrote a little test-program to explain the problem. If required, I
> will post the complete test-program, but for now I will show some
> code-snippets.
> 
> In the user-space application a semphore is created using the sem_open sys-call:
> 
>   clockSem = sem_open(SEMNAME, O_CREAT, 0, 0);
> 
> clockSem, which is a pointer to sem_t returns an address (!= SEM_FAILED).
> With an ioctl-call I try to open the same semaphore in kernel-mode:
> 
>   sem_p = sem_open(SEMNAME, 0);
> 
> This fails. sem_p equals SEM_FAILED, and errno is set to 2 (ENOENT).
> 
> I'm runing Linux 2.6.29.4 x86 32bit, Xenomai 2.4.8, ipipe 2.4-01
> 

The proper design is to use a rtdm semaphore or event *inside* the
kernel and let the application block on it when it calls a related IOCTL
or other device function that shall synchronize on that condition.

Don't share resources like this between kernel and user space. It's
unclean, specifically as you once may want to access driver managed data
along the sleeping/waking as well.

Jan

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