linux-kernel - Re: [RFC][PATCH 5/8] rtmutex: Clean up

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Date:	Tue, 14 Jun 2016 13:08:13 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, tglx@...utronix.de, rostedt@...dmis.org,
	xlpang@...hat.com, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...icios.com, jdesfossez@...icios.com,
	bristot@...hat.com
Subject: Re: [RFC][PATCH 5/8] rtmutex: Clean up

Hi,

On 07/06/16 21:56, Peter Zijlstra wrote:
> Previous patches changed the meaning of the return value of
> rt_mutex_slowunlock(); update comments and code to reflect this.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/futex.c                  |   12 ++++++------
>  kernel/locking/rtmutex.c        |   20 +++++++++-----------
>  kernel/locking/rtmutex_common.h |    2 +-
>  3 files changed, 16 insertions(+), 18 deletions(-)
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1261,7 +1261,7 @@ static int wake_futex_pi(u32 __user *uad
>  	struct futex_pi_state *pi_state = this->pi_state;
>  	u32 uninitialized_var(curval), newval;
>  	WAKE_Q(wake_q);
> -	bool deboost;
> +	bool postunlock;
>  	int ret = 0;
>  
>  	if (!pi_state)
> @@ -1327,17 +1327,17 @@ static int wake_futex_pi(u32 __user *uad
>  
>  	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
>  
> -	deboost = rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
> +	postunlock = rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
>  
>  	/*
>  	 * First unlock HB so the waiter does not spin on it once he got woken
> -	 * up. Second wake up the waiter before the priority is adjusted. If we
> -	 * deboost first (and lose our higher priority), then the task might get
> -	 * scheduled away before the wake up can take place.
> +	 * up. Then wakeup the waiter by calling rt_mutex_postunlock(). Priority
> +	 * is already adjusted and preemption is disabled to avoid inversion.
>  	 */
>  	spin_unlock(&hb->lock);
>  
> -	rt_mutex_postunlock(&wake_q, deboost);
> +	if (postunlock)
> +		rt_mutex_postunlock(&wake_q);

I'm most probably missing something, but don't we still need to call
wake_up_q() even when postunlock is false? IIUC, we were always doing
that, rt_mutex_postunlock(), before this change (only calling
preempt_enable() was conditional).

Best,

- Juri

>  
>  	return 0;
>  }
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1254,7 +1254,8 @@ static inline int rt_mutex_slowtrylock(s
>  
>  /*
>   * Slow path to release a rt-mutex.
> - * Return whether the current task needs to undo a potential priority boosting.
> + *
> + * Return whether the current task needs to call rt_mutex_postunlock().
>   */
>  static bool __sched rt_mutex_slowunlock(struct rt_mutex *lock,
>  					struct wake_q_head *wake_q)
> @@ -1327,7 +1328,7 @@ static bool __sched rt_mutex_slowunlock(
>  
>  	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>  
> -	/* check PI boosting */
> +	/* call rt_mutex_postunlock() */
>  	return true;
>  }
>  
> @@ -1378,15 +1379,14 @@ rt_mutex_fasttrylock(struct rt_mutex *lo
>  }
>  
>  /*
> - * Undo pi boosting (if necessary) and wake top waiter.
> + * Performs the wakeup of the the top-waiter and re-enables preemption.
>   */
> -void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
> +void rt_mutex_postunlock(struct wake_q_head *wake_q)
>  {
>  	wake_up_q(wake_q);
>  
>  	/* Pairs with preempt_disable() in rt_mutex_slowunlock() */
> -	if (deboost)
> -		preempt_enable();
> +	preempt_enable();
>  }
>  
>  /**
> @@ -1489,9 +1489,8 @@ void __sched rt_mutex_unlock(struct rt_m
>  		rt_mutex_deadlock_account_unlock(current);
>  
>  	} else {
> -		bool deboost = rt_mutex_slowunlock(lock, &wake_q);
> -
> -		rt_mutex_postunlock(&wake_q, deboost);
> +		if (rt_mutex_slowunlock(lock, &wake_q))
> +			rt_mutex_postunlock(&wake_q);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(rt_mutex_unlock);
> @@ -1500,8 +1499,7 @@ EXPORT_SYMBOL_GPL(rt_mutex_unlock);
>   * rt_mutex_futex_unlock - Futex variant of rt_mutex_unlock
>   * @lock: the rt_mutex to be unlocked
>   *
> - * Returns: true/false indicating whether priority adjustment is
> - * required or not.
> + * Returns: true/false indicating whether we should call rt_mutex_postunlock().
>   */
>  bool __sched rt_mutex_futex_unlock(struct rt_mutex *lock,
>  				   struct wake_q_head *wqh)
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -111,7 +111,7 @@ extern int rt_mutex_finish_proxy_lock(st
>  extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to);
>  extern bool rt_mutex_futex_unlock(struct rt_mutex *lock,
>  				  struct wake_q_head *wqh);
> -extern void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost);
> +extern void rt_mutex_postunlock(struct wake_q_head *wake_q);
>  extern void rt_mutex_adjust_prio(struct task_struct *task);
>  
>  #ifdef CONFIG_DEBUG_RT_MUTEXES
> 
>