linux-kernel - Re: [PATCH -rfc 4/4] locking/rtmutex: Support spin on owner (osq)

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Message-ID: <alpine.DEB.2.11.1506051443250.7723@nanos>
Date:	Fri, 5 Jun 2015 15:59:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH -rfc 4/4] locking/rtmutex: Support spin on owner (osq)

On Tue, 19 May 2015, Davidlohr Bueso wrote:
>  
> +/*
> + * Lockless alternative to rt_mutex_has_waiters() as we do not need the
> + * wait_lock to check if we are in, for instance, a transitional state
> + * after calling mark_rt_mutex_waiters().

Before I get into a state of brain melt, could you please explain that
in an understandable way? 

rt_mutex_has_waiters() looks at the root pointer of the rbtree head
whether that's empty. You can do a lockless check of that as well,
right? So what's the FAST part of that function and how is that
related to a point after we called mark_rt_mutex_waiters()?

> + */
> +static inline bool rt_mutex_has_waiters_fast(struct rt_mutex *lock)
> +{
> +	unsigned long val = (unsigned long)lock->owner;
> +
> +	if (!val)
> +		return false;
> +	return val & RT_MUTEX_HAS_WAITERS;
> +}
> +

> +/*
> + * Initial check for entering the mutex spinning loop
> + */
> +static inline bool rt_mutex_can_spin_on_owner(struct rt_mutex *lock)
> +{
> +	struct task_struct *owner;
> +	/* default return to spin: if no owner, the lock is free */


Rather than having a comment in the middle of the variable declaration
section, I'd prefer a comment explaing the whole logic of this
function.

> +	int ret = true;

> +static bool rt_mutex_optimistic_spin(struct rt_mutex *lock)
> +{
> +	bool taken = false;
> +
> +	preempt_disable();
> +
> +	if (!rt_mutex_can_spin_on_owner(lock))
> +		goto done;
> +	/*
> +	 * In order to avoid a stampede of mutex spinners trying to
> +	 * acquire the mutex all at once, the spinners need to take a
> +	 * MCS (queued) lock first before spinning on the owner field.
> +	 */
> +	if (!osq_lock(&lock->osq))
> +		goto done;

Hmm. The queue lock is serializing potential spinners, right?

So that's going to lead to a potential priority ordering problem
because if a lower prio task wins the racing to the ocq_lock queue,
then the higher prio waiter will be queued behind and blocked from
taking the lock first.

Thanks,

	tglx
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