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Subject[patch 3/6] rtmutex: Cleanup deadlock detector debug logic
The conditions under which deadlock detection is conducted are unclear
and undocumented.

Add constants instead of using 0/1 and provide a selection function
which hides the additional debug dependency from the calling code.

Add comments where needed.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/futex.c | 13 ++++++++-----
kernel/locking/rtmutex.c | 37 ++++++++++++++++++++++++++++++-------
kernel/locking/rtmutex.h | 7 ++++++-
kernel/locking/rtmutex_common.h | 15 +++++++++++++++
4 files changed, 59 insertions(+), 13 deletions(-)

Index: tip/kernel/futex.c
===================================================================
--- tip.orig/kernel/futex.c
+++ tip/kernel/futex.c
@@ -1619,7 +1619,8 @@ retry_private:
this->pi_state = pi_state;
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
- this->task, 1);
+ this->task,
+ RT_MUTEX_FULL_CHAINWALK);
if (ret == 1) {
/* We got the lock. */
requeue_pi_wake_futex(this, &key2, hb2);
@@ -2238,9 +2239,10 @@ retry_private:
/*
* Block on the PI mutex:
*/
- if (!trylock)
- ret = rt_mutex_timed_lock(&q.pi_state->pi_mutex, to, 1);
- else {
+ if (!trylock) {
+ ret = rt_mutex_timed_lock(&q.pi_state->pi_mutex, to,
+ RT_MUTEX_FULL_CHAINWALK);
+ } else {
ret = rt_mutex_trylock(&q.pi_state->pi_mutex);
/* Fixup the trylock return value: */
ret = ret ? 0 : -EWOULDBLOCK;
@@ -2562,7 +2564,8 @@ static int futex_wait_requeue_pi(u32 __u
*/
WARN_ON(!q.pi_state);
pi_mutex = &q.pi_state->pi_mutex;
- ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1);
+ ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter,
+ RT_MUTEX_FULL_CHAINWALK);
debug_rt_mutex_free_waiter(&rt_waiter);

spin_lock(q.lock_ptr);
Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -256,6 +256,25 @@ static void rt_mutex_adjust_prio(struct
}

/*
+ * Deadlock detection is conditional:
+ *
+ * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
+ * if the detect argument is != 0.
+ *
+ * If CONFIG_DEBUG_RT_MUTEXES=y, deadlock detection is always
+ * conducted independent of the detect argument.
+ *
+ * If the waiter argument is NULL this indicates the deboost path and
+ * deadlock detection is disabled independent of the detect argument
+ * and the config settings.
+ */
+static int
+rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter, int detect)
+{
+ return debug_rt_mutex_detect_deadlock(waiter, detect);
+}
+
+/*
* Max number of times we'll walk the boosting chain:
*/
int max_lock_depth = 1024;
@@ -289,8 +308,8 @@ static int rt_mutex_adjust_prio_chain(st
int detect_deadlock, ret = 0, depth = 0;
unsigned long flags;

- detect_deadlock = debug_rt_mutex_detect_deadlock(orig_waiter,
- deadlock_detect);
+ detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter,
+ deadlock_detect);

/*
* The (de)boosting is a step by step approach with a lot of
@@ -569,9 +588,10 @@ static int task_blocks_on_rt_mutex(struc
if (owner->pi_blocked_on)
chain_walk = 1;
raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
- }
- else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock))
+
+ } else if (rt_mutex_cond_detect_deadlock(waiter, detect_deadlock)) {
chain_walk = 1;
+ }

if (!chain_walk)
return 0;
@@ -674,7 +694,8 @@ static void remove_waiter(struct rt_mute

raw_spin_unlock(&lock->wait_lock);

- rt_mutex_adjust_prio_chain(owner, 0, lock, NULL, current);
+ rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
+ NULL, current);

raw_spin_lock(&lock->wait_lock);
}
@@ -702,7 +723,8 @@ void rt_mutex_adjust_pi(struct task_stru

/* gets dropped in rt_mutex_adjust_prio_chain()! */
get_task_struct(task);
- rt_mutex_adjust_prio_chain(task, 0, NULL, NULL, task);
+ rt_mutex_adjust_prio_chain(task, RT_MUTEX_MIN_CHAINWALK, NULL, NULL,
+ task);
}

/**
@@ -928,7 +950,8 @@ void __sched rt_mutex_lock(struct rt_mut
{
might_sleep();

- rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, 0, rt_mutex_slowlock);
+ rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, RT_MUTEX_MIN_CHAINWALK,
+ rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_lock);

Index: tip/kernel/locking/rtmutex.h
===================================================================
--- tip.orig/kernel/locking/rtmutex.h
+++ tip/kernel/locking/rtmutex.h
@@ -22,5 +22,10 @@
#define debug_rt_mutex_init(m, n) do { } while (0)
#define debug_rt_mutex_deadlock(d, a ,l) do { } while (0)
#define debug_rt_mutex_print_deadlock(w) do { } while (0)
-#define debug_rt_mutex_detect_deadlock(w,d) (d)
#define debug_rt_mutex_reset_waiter(w) do { } while (0)
+
+static inline int debug_rt_mutex_detect_deadlock(struct rt_mutex_waiter *waiter,
+ int detect)
+{
+ return detect == RT_MUTEX_FULL_CHAINWALK;
+}
Index: tip/kernel/locking/rtmutex_common.h
===================================================================
--- tip.orig/kernel/locking/rtmutex_common.h
+++ tip/kernel/locking/rtmutex_common.h
@@ -80,6 +80,21 @@ static inline struct task_struct *rt_mut
}

/*
+ * Constants for rt mutex functions which have a selectable deadlock
+ * detection.
+ *
+ * RT_MUTEX_MIN_CHAINWALK: Stops the lock chain walk when there are
+ * no further PI adjustments to be made.
+ *
+ * RT_MUTEX_FULL_CHAINWALK: Invoke deadlock detection with a full
+ * walk of the lock chain.
+ */
+enum {
+ RT_MUTEX_MIN_CHAINWALK,
+ RT_MUTEX_FULL_CHAINWALK,
+};
+
+/*
* PI-futex support (proxy locking functions, etc.):
*/
extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);



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