locking/rtmutex: Acquire the hb lock via trylock after wait-proxylock.

After rt_mutex_wait_proxy_lock() task_struct::pi_blocked_on is cleared
if current owns the lock. If the operation has been interrupted by a
signal or timeout then pi_blocked_on can be set. This means spin_lock()
*can* overwrite pi_blocked_on on PREEMPT_RT. This has been noticed by
the recently added lockdep-asserts…

The rt_mutex_cleanup_proxy_lock() operation will clear pi_blocked_on
(and update pending waiters as expected) but it must happen under the hb
lock to ensure the same state in rtmutex and userland.

Given all the possibilities it is probably the simplest option to
try-lock the hb lock. In case the lock is occupied a quick nap is
needed. A busy loop can lock up the system if performed by a task with
high priorioty preventing the owner from running.

The rt_mutex_post_schedule() needs to be put before try-lock-loop
because otherwie the schedule() in schedule_hrtimeout() will trip over
the !sched_rt_mutex assert.

Introduce futex_trylock_hblock() to try-lock the hb lock and sleep until
the try-lock operation succeeds. Use it after rt_mutex_wait_proxy_lock()
to acquire the lock.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230831095314.fTliy0Bh@linutronix.de


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