Philippe Gerum authored Mar 14, 2021 and Jan Kiszka committed Jun 11, 2021

Dovetail provides a fast service to escalate the caller to out-of-band
mode for executing a routine (run_oob_call()), which we use to enforce
primary mode in ___xnsched_run() to schedule out the relaxing thread.

Due to the way run_oob_call() works, enabling hardirqs during this
transition can trigger a subtle bug caused by the relaxing thread to
be switched out, as a result of taking an interrupt during the irqs on
section, before __xnsched_run() actually runs on behalf of
xnthread_relax() -> xnthread_suspend().

This may lead to a breakage of the inband interrupt state, revealed by
lockdep complaining about a HARDIRQ-IN-W -> HARDIRQ-ON-W situation,
when finalize_task_switch() runs for reconciling both the in-band and
Xenomai scheduler states.

Re-enabling hard irqs before switching out the relaxing thread was
throught as a mean to reduce the scope of the interrupt-free section
while relaxing a thread with the I-pipe, which unlike Dovetail
requires us to open code an escalation service based on triggering a
synthetic interrupt.

We differentiate the behavior between the I-pipe and Dovetail by
abstracting the related bits as pipeline_leave_oob_unlock(), keeping
the current implementation unchanged for the I-pipe.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>