linux-kernel - Re: [patch 26/50] locking/rtmutex: Provide the spin/rwlock core lock function

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Date:   Wed, 14 Jul 2021 12:59:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [patch 26/50] locking/rtmutex: Provide the spin/rwlock core lock
 function

On Tue, Jul 13, 2021 at 05:11:20PM +0200, Thomas Gleixner wrote:
> +/**
> + * rtlock_slowlock_locked - Slow path lock acquisition for RT locks
> + * @lock:	The underlying rt mutex
> + */
> +static void __sched rtlock_slowlock_locked(struct rt_mutex *lock)
> +{
> +	struct rt_mutex_waiter waiter;
> +
> +	lockdep_assert_held(&lock->wait_lock);
> +
> +	if (try_to_take_rt_mutex(lock, current, NULL))
> +		return;
> +
> +	rt_mutex_init_rtlock_waiter(&waiter);
> +
> +	/* Save current state and set state to TASK_RTLOCK_WAIT */
> +	current_save_and_set_rtlock_wait_state();
> +
> +	task_blocks_on_rt_mutex(lock, &waiter, current, RT_MUTEX_MIN_CHAINWALK);
> +
> +	for (;;) {
> +		/* Try to acquire the lock again. */
> +		if (try_to_take_rt_mutex(lock, current, &waiter))
> +			break;
> +
> +		raw_spin_unlock_irq(&lock->wait_lock);
> +
> +		schedule_rtlock();
> +
> +		raw_spin_lock_irq(&lock->wait_lock);
> +		set_current_state(TASK_RTLOCK_WAIT);

Since both waker and waiter are fully serialized on ->wait_lock, this
can be __set_current_state(), there is no race condition AFAICT.

Might not be worth it though..

> +	}
> +
> +	/* Restore the task state */
> +	current_restore_rtlock_saved_state();
> +
> +	/*
> +	 * try_to_take_rt_mutex() sets the waiter bit unconditionally. We
> +	 * might have to fix that up:
> +	 */
> +	fixup_rt_mutex_waiters(lock);
> +	debug_rt_mutex_free_waiter(&waiter);
> +}