[PATCH RT 06/36] rtmutex: Clarify the boost/deboost part

Steven Rostedt
2015-03-12T19:36:57Z

[-- Attachment #1: 0006-rtmutex-Clarify-the-boost-deboost-part.patch --]
[-- Type: text/plain, Size: 4763 bytes --]

3.12.38-rt53-rc1 stable review patch.
If anyone has any objections, please let me know.

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

From: Thomas Gleixner <tglx@linutronix.de>

upstream commit: a57594a13a446d1a6ab1dcd48339f799ce586843

Add a separate local variable for the boost/deboost logic to make the
code more readable. Add comments where appropriate.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Conflicts:
kernel/rtmutex.c
---
kernel/rtmutex.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 48 insertions(+), 9 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 7a089565d869..5e3029f936e9 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -292,9 +292,10 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
struct rt_mutex_waiter *orig_waiter,
struct task_struct *top_task)
{
- struct rt_mutex *lock;
struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter;
+ struct rt_mutex_waiter *prerequeue_top_waiter;
int detect_deadlock, ret = 0, depth = 0;
+ struct rt_mutex *lock;
unsigned long flags;

detect_deadlock = debug_rt_mutex_detect_deadlock(orig_waiter,
@@ -401,9 +402,14 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
goto out_unlock_pi;
}

- top_waiter = rt_mutex_top_waiter(lock);
+ /*
+ * Store the current top waiter before doing the requeue
+ * operation on @lock. We need it for the boost/deboost
+ * decision below.
+ */
+ prerequeue_top_waiter = rt_mutex_top_waiter(lock);

- /* Requeue the waiter */
+ /* Requeue the waiter in the lock waiter list. */
rt_mutex_dequeue(lock, waiter);
waiter->list_entry.prio = task->prio;
rt_mutex_enqueue(lock, waiter);
@@ -412,6 +418,11 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
put_task_struct(task);

+ /*
+ * 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.
+ */
if (!rt_mutex_owner(lock)) {
struct rt_mutex_waiter *lock_top_waiter;

@@ -420,29 +431,48 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
* to wake the new top waiter up to try to get the lock.
*/
lock_top_waiter = rt_mutex_top_waiter(lock);
- if (top_waiter != lock_top_waiter)
+ if (prerequeue_top_waiter != lock_top_waiter)
rt_mutex_wake_waiter(lock_top_waiter);
raw_spin_unlock(&lock->wait_lock);
return 0;
}

- /* Grab the next task */
+ /* 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);

if (waiter == rt_mutex_top_waiter(lock)) {
- /* Boost the owner */
- rt_mutex_dequeue_pi(task, top_waiter);
+ /*
+ * The waiter became the new top (highest priority)
+ * waiter on the lock. Replace the previous top waiter
+ * in the owner tasks pi waiters list with this waiter
+ * and adjust the priority of the owner.
+ */
+ rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
rt_mutex_enqueue_pi(task, waiter);
__rt_mutex_adjust_prio(task);

- } else if (top_waiter == waiter) {
- /* Deboost the owner */
+ } else if (prerequeue_top_waiter == waiter) {
+ /*
+ * The waiter was the top waiter on the lock, but is
+ * no longer the top prority waiter. Replace waiter in
+ * the owner tasks pi waiters list with the new top
+ * (highest priority) waiter and adjust the priority
+ * of the owner.
+ * The new top waiter is stored in @waiter so that
+ * @waiter == @top_waiter evaluates to true below and
+ * we continue to deboost the rest of the chain.
+ */
rt_mutex_dequeue_pi(task, waiter);
waiter = rt_mutex_top_waiter(lock);
rt_mutex_enqueue_pi(task, waiter);
__rt_mutex_adjust_prio(task);
+ } else {
+ /*
+ * Nothing changed. No need to do any priority
+ * adjustment.
+ */
}

/*
@@ -455,6 +485,10 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *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);
raw_spin_unlock(&lock->wait_lock);

@@ -465,6 +499,11 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
if (!next_lock)
goto out_put_task;

+ /*
+ * If the current waiter is not the top waiter on the lock,
+ * then we can stop the chain walk here if we are not in full
+ * deadlock detection mode.
+ */
if (!detect_deadlock && waiter != top_waiter)
goto out_put_task;

--
2.1.4