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
+ *
+ * call() Protected by
+ * @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]
+ * check_exit_conditions(); [C]
+ * task = owner(lock); [C]
+ * get_task_ref(task); [C] acquire [A]
+ * 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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