On Mon, 09 Jun 2014 20:28:08 -0000
Thomas Gleixner <t...@linutronix.de> wrote:
> Add commentry to document the chain walk and the protection mechanisms
> and their scope.
>
> Signed-off-by: Thomas Gleixner <t...@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);
>
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