Re: [Xenomai] Prio ceiling mutex issues

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2016-05-09 21:03, Jan Kiszka wrote:
> Hi Philippe,
> 
> we ran into at least one bug of the current prio ceiling support for
> synch objects: on xnsynch_release, we only reschedule if we woken up a
> new owner. But we also have to reschedule after dropping the ceiling
> prio as some higher prio thread may be waiting.
> 
> For that, I would propose to alter the return type and semantic of
> xnsynch_release from "pointer to new owner" into "bool, true if
> reschedule is needed". It seems, no caller of xnsynch_release makes a
> non-boolean use of the return value anyway. And then we can simply
> return true on "synch->status & XNSYNCH_PP".
> 
> That leads to the question what would happen in the fast case. Consider
> 
> pthread_mutex_lock(ceiling_mutex);
> wake_up_thread(some_thread); // caller_prio < target_prio < ceiling_prio
> pthread_mutex_unlock(ceiling_mutex);
> 
> To my current understanding, there is nothing enforcing a syscall for
> the unlock caller if some thread woke up after the lock, thus may have a
> high prio after the unlock. Or am I missing something?

OK, I think I got it: When we found a reason to reschedule after taking
such a ceiling lock lazily, we commit the prio-boost and enforce the
kernel exit on release via xnsynch_fast_ceiling in commit_ceiling,
right? Then we only need to fix the above scheduling issue and are fine.
I'll quickly hack up some patches for discussion.

Jan

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