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Subject[PATCH RT 07/25] rtmutex: Document pi chain walk
3.2.68-rt99-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@linutronix.de>

upstream commit: 3eb65aeadf701976b084e9171e16bb7d1e83fbb0

Add commentry to document the chain walk and the protection mechanisms
and their scope.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/rtmutex.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 91 insertions(+), 9 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 34c63cb7ff22..5c581a7b715f 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -281,6 +281,48 @@ static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
* @top_task: the current top waiter
*
* Returns 0 or -EDEADLK.
+ *
+ * Chain walk basics and protection scope
+ *
+ * [R] refcount on task
+ * [P] task->pi_lock held
+ * [L] rtmutex->wait_lock held
+ *
+ * Step Description Protected by
+ * function arguments:
+ * @task [R]
+ * @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:
+ * [1] lock(task->pi_lock); [R] acquire [P]
+ * [2] waiter = task->pi_blocked_on; [P]
+ * [3] check_exit_conditions_1(); [P]
+ * [4] lock = waiter->lock; [P]
+ * [5] if (!try_lock(lock->wait_lock)) { [P] try to acquire [L]
+ * unlock(task->pi_lock); release [P]
+ * goto retry;
+ * }
+ * [6] check_exit_conditions_2(); [P] + [L]
+ * [7] requeue_lock_waiter(lock, waiter); [P] + [L]
+ * [8] unlock(task->pi_lock); release [P]
+ * put_task_struct(task); release [R]
+ * [9] check_exit_conditions_3(); [L]
+ * [10] task = owner(lock); [L]
+ * get_task_struct(task); [L] acquire [R]
+ * lock(task->pi_lock); [L] acquire [P]
+ * [11] requeue_pi_waiter(tsk, waiters(lock));[P] + [L]
+ * [12] check_exit_conditions_4(); [P] + [L]
+ * [13] unlock(task->pi_lock); release [P]
+ * unlock(lock->wait_lock); release [L]
+ * goto again;
*/
static int rt_mutex_adjust_prio_chain(struct task_struct *task,
int deadlock_detect,
@@ -305,6 +347,9 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
* carefully whether things change under us.
*/
again:
+ /*
+ * We limit the lock chain length for each invocation.
+ */
if (++depth > max_lock_depth) {
static int prev_max;

@@ -322,13 +367,28 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,

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/again) dropped all
+ * locks.
+ */
retry:
/*
- * Task can not go away as we did a get_task() before !
+ * [1] Task cannot go away as we did a get_task() before !
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);

+ /*
+ * [2] Get the waiter on which @task is blocked on.
+ */
waiter = task->pi_blocked_on;
+
+ /*
+ * [3] check_exit_conditions_1() protected by task->pi_lock.
+ */
+
/*
* Check whether the end of the boosting chain has been
* reached or the state of the chain has changed while we
@@ -379,7 +439,15 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
if (!detect_deadlock && waiter->list_entry.prio == task->prio)
goto out_unlock_pi;

+ /*
+ * [4] Get the next lock
+ */
lock = waiter->lock;
+ /*
+ * [5] We need to trylock here as we are holding task->pi_lock,
+ * which is the reverse lock order versus the other rtmutex
+ * operations.
+ */
if (!raw_spin_trylock(&lock->wait_lock)) {
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
cpu_relax();
@@ -387,6 +455,9 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
}

/*
+ * [6] check_exit_conditions_2() protected by task->pi_lock and
+ * lock->wait_lock.
+ *
* Deadlock detection. If the lock is the same as the original
* lock which caused us to walk the lock chain or if the
* current lock is owned by the task which initiated the chain
@@ -406,16 +477,18 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
*/
prerequeue_top_waiter = rt_mutex_top_waiter(lock);

- /* Requeue the waiter in the lock waiter list. */
+ /* [7] Requeue the waiter in the lock waiter list. */
rt_mutex_dequeue(lock, waiter);
waiter->list_entry.prio = task->prio;
rt_mutex_enqueue(lock, waiter);

- /* Release the task */
+ /* [8] Release the task */
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
put_task_struct(task);

/*
+ * [9] check_exit_conditions_3 protected by lock->wait_lock.
+ *
* We must abort the chain walk if there is no lock owner even
* in the dead lock detection case, as we have nothing to
* follow here. This is the end of the chain we are walking.
@@ -424,8 +497,9 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
struct rt_mutex_waiter *lock_top_waiter;

/*
- * If the requeue above changed the top waiter, then we need
- * to wake the new top waiter up to try to get the lock.
+ * If the requeue [7] above changed the top waiter,
+ * then we need to wake the new top waiter up to try
+ * to get the lock.
*/
lock_top_waiter = rt_mutex_top_waiter(lock);
if (prerequeue_top_waiter != lock_top_waiter)
@@ -434,11 +508,12 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
return 0;
}

- /* Grab the next task, i.e. the owner of @lock */
+ /* [10] Grab the next task, i.e. the owner of @lock */
task = rt_mutex_owner(lock);
get_task_struct(task);
raw_spin_lock_irqsave(&task->pi_lock, flags);

+ /* [11] requeue the pi waiters if necessary */
if (waiter == rt_mutex_top_waiter(lock)) {
/*
* The waiter became the new top (highest priority)
@@ -473,23 +548,30 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
}

/*
+ * [12] check_exit_conditions_4() protected by task->pi_lock
+ * and lock->wait_lock. The actual decisions are made after we
+ * dropped the locks.
+ *
* Check whether the task which owns the current lock is pi
* blocked itself. If yes we store a pointer to the lock for
* the lock chain change detection above. After we dropped
* task->pi_lock next_lock cannot be dereferenced anymore.
*/
next_lock = task_blocked_on_lock(task);
-
- raw_spin_unlock_irqrestore(&task->pi_lock, flags);
-
/*
* Store the top waiter of @lock for the end of chain walk
* decision below.
*/
top_waiter = rt_mutex_top_waiter(lock);
+
+ /* [13] Drop the locks */
+ raw_spin_unlock_irqrestore(&task->pi_lock, flags);
raw_spin_unlock(&lock->wait_lock);

/*
+ * Make the actual exit decisions [12], based on the stored
+ * values.
+ *
* We reached the end of the lock chain. Stop right here. No
* point to go back just to figure that out.
*/
--
2.1.4



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