[patch v2 4/5] rtmutex: Clarify the lock chain walk
From: Thomas Gleixner
Date: Sat May 31 2014 - 11:58:00 EST
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@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20140522031950.168005692@xxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
kernel/locking/rtmutex.c | 51 +++++++++++++++++++++++++++++++++++++++--------
1 file changed, 43 insertions(+), 8 deletions(-)
Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -303,10 +303,11 @@ static int rt_mutex_adjust_prio_chain(st
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;
enum rtmutex_chainwalk detect_deadlock;
int ret = 0, depth = 0;
+ struct rt_mutex *lock;
unsigned long flags;
detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter,
@@ -381,6 +382,11 @@ 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);
@@ -401,7 +407,11 @@ static int rt_mutex_adjust_prio_chain(st
goto out_unlock_pi;
}
- top_waiter = rt_mutex_top_waiter(lock);
+ /*
+ * Store the top waiter before doing any requeue operation. We
+ * need it for the boost/deboost decision below.
+ */
+ prerequeue_top_waiter = rt_mutex_top_waiter(lock);
/* Requeue the waiter */
rt_mutex_dequeue(lock, waiter);
@@ -410,13 +420,18 @@ static int rt_mutex_adjust_prio_chain(st
/* Release the task */
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+
+ /*
+ * 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)) {
/*
* 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 (top_waiter != rt_mutex_top_waiter(lock))
+ if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
wake_up_process(rt_mutex_top_waiter(lock)->task);
raw_spin_unlock(&lock->wait_lock);
goto out_put_task;
@@ -429,17 +444,32 @@ static int rt_mutex_adjust_prio_chain(st
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.
+ */
+ 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.
+ */
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.
+ */
}
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
@@ -447,6 +477,11 @@ static int rt_mutex_adjust_prio_chain(st
top_waiter = rt_mutex_top_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
+ /*
+ * 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;
--
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