Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock after wait-proxylock.

[Peter Zijlstra]

On Fri, Sep 08, 2023 at 06:22:54PM +0200, Sebastian Andrzej Siewior wrote:
> 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.
> 

Arggggh... this just keeps getting better :/