Re: Spurious task resume due to SIGSHADOW

On 12.05.26 09:19, Richard Weinberger wrote:
> On Tue, May 12, 2026 at 7:50 AM Jan Kiszka <[email protected]> wrote:
>>
>> On 11.05.26 23:49, Richard Weinberger wrote:
>>> I'm currently debugging an issue on a very old Xenomai system, while
>>> reading the source I noticed that the very same issue could also exist
>>> on recent Xenomai releases.
>>>
>>> What I see is the following:
>>> - A task gets suspended and unblocked using rt_task_suspend() and
>>> rt_task_unblock()
>>
>> This is intentionally rt_task_unblock instead of rt_task_resume?
> 
> From what I can tell, yes.
> 
>> Because:
>>
>> "A nesting count is maintained so that rt_task_suspend() and
>> rt_task_resume() must be used in pairs."
> 
> The application has non-trivial thread management, from what I can
> tell, it's fine.
> 

Unlikely, because it will completely confuse tcb->suspends, preventing
future suspends for the task that was not properly resumed.

>>> - In some cases it happens that the task resumes to execute
>>> - It resumes because a Linux signal is pending, being SIGSHADOW
>>>
>>> Since SIGSHADOW is internally used to suspend a task, this alerted me.
>>> So, I suspect the following issue:
>>> When xnthread_suspend() fires SIGSHADOW using
>>> __xnthread_signal(thread, SIGSHADOW, SIGSHADOW_ACTION_HARDEN); nothing
>>> seems to prevent the case where the target task enters the primary
>>> domain right before it sees SIGSHADOW. In the next step
>>> do_sigwake_event() causes it to resume instead of sleeping.
>>>
>>> Hmm?
>>>
>>
>> Traces to confirm this theory would be good. But it could indeed be
>> problematic that the target state (__THREAD_S_SUSPENDED) is only tracked
>> in userspace where the scheduler has no access to while waking a task.
> 
> I think I found a reproducer. See next patch in this mail thread.
> 
> This quick and dirty change seems to mitigate the problem, but I consider it
> a sledge hammer:
> diff --git a/kernel/cobalt/dovetail/kevents.c b/kernel/cobalt/dovetail/kevents.c
> index 7402e2ef6..d72c3788b 100644
> --- a/kernel/cobalt/dovetail/kevents.c
> +++ b/kernel/cobalt/dovetail/kevents.c
> @@ -145,6 +145,14 @@ static int handle_sigwake_event(struct task_struct *p)
>        if (xnthread_test_state(thread, XNRELAX))
>                goto out;
> 
> +       if (sigismember(&p->pending.signal, SIGSHADOW))
> +               goto out;
> +
> +       if (sigismember(&p->signal->shared_pending.signal, SIGSHADOW))
> +               goto out;
> +
>        /*
>         * Allow a thread stopped for debugging to resume briefly in order to
>         * migrate to secondary mode. xnthread_relax will reapply XNDBGSTOP.

Will have a look, thanks.

Jan

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