[Xenomai] Prio ceiling mutex issues

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

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?

Thanks,
Jan

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