Re: [patch V3 3/7] rtmutex: Document pi chain walk

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 09 Jun 2014 20:28:08 -0000
Thomas Gleixner <tglx@linutronix.de> wrote:

> Add commentry to document the chain walk and the protection mechanisms
> and their scope.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  kernel/locking/rtmutex.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> Index: tip/kernel/locking/rtmutex.c
> ===================================================================
> --- tip.orig/kernel/locking/rtmutex.c
> +++ tip/kernel/locking/rtmutex.c
> @@ -285,6 +285,47 @@ static inline struct rt_mutex *task_bloc
>   * @top_task:	the current top waiter
>   *
>   * Returns 0 or -EDEADLK.
> + *
> + * Chain walk basics and protection scope
> + *
> + * [A] refcount on task
> + * [B] task->pi_lock held
> + * [C] rtmutex->lock held

A,B, C is rather meaningless, and requires constant looking back up at
the key. Perhaps [R],[P] and [L]

 [R] refcount on task (get_task_struct)
 [P] task->pi_lock held
 [L] rtmutex->lock held


That way we can associate R being refcount, P being pi_lock and L being
lock. Easier to remember.


> + *
> + * call()					Protected by

"call()"?

> + *	@task					[A]
> + *	@orig_lock if != NULL			@top_task is blocked on it
> + *	@next_lock				Unprotected. Cannot be
> + *						dereferenced. Only used for
> + *						comparison.
> + *	@orig_waiter if != NULL			@top_task is blocked on it
> + *	@top_task				current, or in case of proxy
> + *						locking protected by calling
> + *						code
> + * again:
> + *	loop_sanity_check();
> + * retry:
> + *	lock(task->pi_lock);			[A] acquire [B]
> + *	waiter = task->pi_blocked_on;		[B]
> + *	check_exit_conditions();		[B]
> + *	lock = waiter->lock;			[B]
> + *	if (!try_lock(lock->wait_lock)) {	[B] try to acquire [C]
> + *		unlock(task->pi_lock);		drop [B]
> + *		goto retry;
> + *	}
> + *	check_exit_conditions();		[B] + [C]
> + *	requeue_lock_waiter(lock, waiter);	[B] + [C]
> + *	unlock(task->pi_lock);			drop [B]
> + *	drop_task_ref(task);			drop [A]

Maybe just state "put_task_struct()", less abstractions.

> + *	check_exit_conditions();		[C]
> + *	task = owner(lock);			[C]
> + *	get_task_ref(task);			[C] acquire [A]

get_task_struct()

-- Steve

> + *	lock(task->pi_lock);			[C] acquire [B]
> + *	requeue_pi_waiter(task, waiters(lock));	[B] + [C]
> + *	check_exit_conditions();		[B] + [C]
> + *	unlock(task->pi_lock);			drop [B]
> + *	unlock(lock->wait_lock);		drop [C]
> + *	goto again;
>   */
>  static int rt_mutex_adjust_prio_chain(struct task_struct *task,
>  				      int deadlock_detect,
> @@ -326,6 +367,12 @@ static int rt_mutex_adjust_prio_chain(st
>  
>  		return -EDEADLK;
>  	}
> +
> +	/*
> +	 * We are fully preemptible here and only hold the refcount on
> +	 * @task. So everything can have changed under us since the
> +	 * caller or our own code below (goto retry) dropped all locks.
> +	 */
>   retry:
>  	/*
>  	 * Task can not go away as we did a get_task() before !
> @@ -383,6 +430,11 @@ static int rt_mutex_adjust_prio_chain(st
>  	if (!detect_deadlock && waiter->prio == task->prio)
>  		goto out_unlock_pi;
>  
> +	/*
> +	 * We need to trylock here as we are holding task->pi_lock,
> +	 * which is the reverse lock order versus the other rtmutex
> +	 * operations.
> +	 */
>  	lock = waiter->lock;
>  	if (!raw_spin_trylock(&lock->wait_lock)) {
>  		raw_spin_unlock_irqrestore(&task->pi_lock, flags);
>