On 06/10/2014 08:45 AM, Steven Rostedt wrote:
> 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;
>> */
There are four check_exit_conditions()s with the same name but with different
locking.
I don't think it is a good a idea to copy the code to the comment of
the function description, we will need to always keep them coincident forever.
I prefer to comment them in the function body or comment them
in higher level abstraction.
>> 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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