linux-kernel - Re: [patch V5 32/72] locking/rtmutex: Provide the spin/rwlock core lock function

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Date:   Sat, 28 Aug 2021 01:21:46 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        Mike Galbraith <efault@....de>
Subject: Re: [patch V5 32/72] locking/rtmutex: Provide the spin/rwlock core
 lock function

On Sun, Aug 15, 2021 at 11:28:25PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> A simplified version of the rtmutex slowlock function which neither handles
> signals nor timeouts and is careful about preserving the state of the
> blocked task across the lock operation.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/locking/rtmutex.c        |   60 ++++++++++++++++++++++++++++++++++++++++
>  kernel/locking/rtmutex_common.h |    2 -
>  2 files changed, 61 insertions(+), 1 deletion(-)
> ---
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1416,3 +1416,63 @@ static __always_inline int __rt_mutex_lo
>  	return rt_mutex_slowlock(lock, state);
>  }
>  #endif /* RT_MUTEX_BUILD_MUTEX */
> +
> +#ifdef RT_MUTEX_BUILD_SPINLOCKS
> +/*
> + * Functions required for spin/rw_lock substitution on RT kernels
> + */
> +
> +/**
> + * rtlock_slowlock_locked - Slow path lock acquisition for RT locks
> + * @lock:	The underlying rt mutex
> + */
> +static void __sched rtlock_slowlock_locked(struct rt_mutex_base *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);
> +	}
> +
> +	/* 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);
> +}
> +
> +static __always_inline void __sched rtlock_slowlock(struct rt_mutex_base *lock)
> +{
> +	unsigned long flags;
> +
> +	raw_spin_lock_irqsave(&lock->wait_lock, flags);
> +	rtlock_slowlock_locked(lock);
> +	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

Just out of curiosity, could we use raw_spin_{un,}lock_irq() here
instead of *_irq{save,restore}()? Because rtlock_slowlock() might sleep,
and we cannot call it with irq-off.

Regards,
Boqun

> +}
> +
> +#endif /* RT_MUTEX_BUILD_SPINLOCKS */
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -181,7 +181,7 @@ static inline void rt_mutex_init_waiter(
>  	waiter->task = NULL;
>  }
>  
> -static inline void rtlock_init_rtmutex_waiter(struct rt_mutex_waiter *waiter)
> +static inline void rt_mutex_init_rtlock_waiter(struct rt_mutex_waiter *waiter)
>  {
>  	rt_mutex_init_waiter(waiter);
>  	waiter->wake_state = TASK_RTLOCK_WAIT;
>