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From: Jan Kiszka <jan.kiszka@siemens.com>
Message-ID: <57602AC8.3070506@siemens.com>
Date: Tue, 14 Jun 2016 18:03:20 +0200
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Subject: Re: [Xenomai] RTDM syscalls & switching
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: Xenomai <xenomai@xenomai.org>

On 2016-06-14 17:51, Gilles Chanteperdrix wrote:
>> Sorry, there *is*: Shadowed thread (anything created by wrapped
>> pthread_create) calls, say, read() on some Linux file descriptor, read()
>> is wrapped, first probes the call on RTDM, which means migration to RT
>> (for currently relaxed threads, like SCHED_WEAK), no RTDM match in the
>> kernel, and finally the migration to NRT in order to do the Linux read()
>> syscall. That didn't happen with the original design.
> 
> The wrapped read does not get ping-pong when calling the Linux I/O
> syscalls. The term syscall means something very precise, and
> __wrap_read is not a syscall. It gets ping-pong because it calls
> RTDM I/O. But if you call directly Linux I/O syscall, with say
> __real_read, you do not get ping-pong. Linux I/O syscall work as
> they have always have: they require xenomai threads to run in
> secondary mode and will cause them to switch to secondary mode to
> handle the syscall.

__real_* are a non-portable workarounds for special cases. It's not what
Xenomai developers are supposed to use in their applications.

If it were, we could also give up on wrapping and have rt_task_create
etc. again, ie. separate APIs for RT and non-RT.

Jan

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