Jan Kiszka wrote:
> Am 05.10.2010 15:42, Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Am 05.10.2010 15:15, Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> Hi,
>>>>> quite a few limitations and complications of using Linux services over
>>>>> non-Linux domains relate to potentially invalid "current" and
>>>>> "thread_info". The non-Linux domain could maintain their own kernel
>>>>> stacks while Linux tend to derive current and thread_info from the stack
>>>>> pointer. This is not an issue anymore on x86-64 (both states are stored
>>>>> in per-cpu variables) but other archs (e.g. x86-32 or ARM) still use the
>>>>> stack and may continue to do so.
>>>>> I just looked into this thing again as I'm evaluating ways to exploit
>>>>> the kernel's tracing framework also under Xenomai. Unfortunately, it
>>>>> does a lot of fiddling with preempt_count and need_resched, so patching
>>>>> it for Xenomai use would become a maintenance nightmare.
>>>>> An alternative, also for other use cases like kgdb and probably perf, is
>>>>> to get rid of our dependency on home-grown stacks. I think we are on
>>>>> that way already as in-kernel skins have been deprecated. The only
>>>>> remaining user after them will be RTDM driver tasks. But I think those
>>>>> could simply become in-kernel shadows of kthreads which would bind their
>>>>> stacks to what Linux provides. Moreover, Xenomai could start updating
>>>>> "current" and "thread_info" on context switches (unless this already
>>>>> happens implicitly). That would give us proper contexts for system-level
>>>>> tracing and profiling.
>>>>> My key question is currently if and how much of this could be realized
>>>>> in 2.6. Could we drop in-kernel skins in that version? If not, what
>>>>> about disabling them by default, converting RTDM tasks to a
>>>>> kthread-based approach, and enabling tracing etc. only in that case?
>>>>> However, this might be a bit fragile unless we can establish
>>>>> compile-time or run-time requirements negotiation between Adeos and its
>>>>> users (Xenomai) about the stack model.
>>>> A stupid question: why not make things the other way around: patch the
>>>> current and current_thread_info functions to be made I-pipe aware and
>>>> use an "ipipe_current" pointer to the current thread task_struct. Of
>>>> course, there are places where the current or current_thread_info macros
>>>> are implemented in assembly, so it may be not simple as it sounds, but
>>>> it would allow to keep 128 Kb stacks if we want. This also means that we
>>>> would have to put a task_struct at the bottom of every Xenomai task.
>>> First of all, overhead vs. maintenance. Either every access to
>>> preempt_count() would require a check for the current domain and its
>>> foreign stack flag, or I would have to patch dozens (if that is enough)
>>> of code sites in the tracer framework.
>> No. I mean we would dereference a pointer named ipipe_current. That is
>> all, no other check. This pointer would be maintained elsewhere. And we
>> modify the "current" macro, like:
>> #ifdef CONFIG_IPIPE
>> extern struct task_struct *ipipe_current;
>> #define current ipipe_current
>> #endif
>> Any calll site gets modified automatically. Or current_thread_info, if
>> it is current_thread_info which is obtained using the stack pointer mask
>> trick.
> The stack pointer mask trick only works with fixed-sized stacks, not a
> guaranteed property of in-kernel Xenomai threads.

Precisely the reason why I propose to replace it with a global variable
reference, or a per-cpu variable for SMP systems.


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