sched/core: Provide sched_rtmutex() and expose sched work helpers

schedule() invokes sched_submit_work() before scheduling and
sched_update_worker() afterwards to ensure that queued block requests are
flushed and the (IO)worker machineries can instantiate new workers if
required. This avoids deadlocks and starvation.

With rt_mutexes this can lead to subtle problem:

  When rtmutex blocks current::pi_blocked_on points to the rtmutex it
  blocks on. When one of the functions in sched_submit/resume_work()
  contends on a rtmutex based lock then that would corrupt
  current::pi_blocked_on.

Make it possible to let rtmutex issue the calls outside of the slowpath,
i.e. when it is guaranteed that current::pi_blocked_on is NULL, by:

  - Exposing sched_submit_work() and moving the task_running() condition
    into schedule()

  - Renamimg sched_update_worker() to sched_resume_work() and exposing it
    too.

  - Providing sched_rtmutex() which just does the inner loop of scheduling
    until need_resched() is not longer set. Split out the loop so this does
    not create yet another copy.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230427111937.2745231-2-bigeasy@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>