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>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/20140522031949.947264874@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

5 files changed, 83 insertions, 28 deletions

diff --git a/kernel/locking/rtmutex-debug.c b/kernel/locking/rtmutex-debug.c
index 49b2ed3dced8..62b6cee8ea7f 100644
--- a/kernel/locking/rtmutex-debug.c
+++ b/kernel/locking/rtmutex-debug.c
@@ -66,12 +66,13 @@ void rt_mutex_debug_task_free(struct task_struct *task)
  * the deadlock. We print when we return. act_waiter can be NULL in
  * case of a remove waiter operation.
  */
-void debug_rt_mutex_deadlock(int detect, struct rt_mutex_waiter *act_waiter,
+void debug_rt_mutex_deadlock(enum rtmutex_chainwalk chwalk,
+                             struct rt_mutex_waiter *act_waiter,
                              struct rt_mutex *lock)
 {
         struct task_struct *task;
 
-        if (!debug_locks || detect || !act_waiter)
+        if (!debug_locks || chwalk == RT_MUTEX_FULL_CHAINWALK || !act_waiter)
                 return;
 
         task = rt_mutex_owner(act_waiter->lock);
diff --git a/kernel/locking/rtmutex-debug.h b/kernel/locking/rtmutex-debug.h
index ab29b6a22669..d0519c3432b6 100644
--- a/kernel/locking/rtmutex-debug.h
+++ b/kernel/locking/rtmutex-debug.h
@@ -20,14 +20,15 @@ extern void debug_rt_mutex_unlock(struct rt_mutex *lock);
 extern void debug_rt_mutex_proxy_lock(struct rt_mutex *lock,
                                       struct task_struct *powner);
 extern void debug_rt_mutex_proxy_unlock(struct rt_mutex *lock);
-extern void debug_rt_mutex_deadlock(int detect, struct rt_mutex_waiter *waiter,
+extern void debug_rt_mutex_deadlock(enum rtmutex_chainwalk chwalk,
+                                    struct rt_mutex_waiter *waiter,
                                     struct rt_mutex *lock);
 extern void debug_rt_mutex_print_deadlock(struct rt_mutex_waiter *waiter);
 # define debug_rt_mutex_reset_waiter(w)                 \
         do { (w)->deadlock_lock = NULL; } while (0)
 
-static inline int debug_rt_mutex_detect_deadlock(struct rt_mutex_waiter *waiter,
-                                                 int detect)
+static inline bool debug_rt_mutex_detect_deadlock(struct rt_mutex_waiter *waiter,
+                                                  enum rtmutex_chainwalk walk)
 {
         return (waiter != NULL);
 }
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 32906482edd1..c6ffdaa21b67 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -308,6 +308,32 @@ static void rt_mutex_adjust_prio(struct task_struct *task)
 }
 
 /*
+ * Deadlock detection is conditional:
+ *
+ * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
+ * if the detect argument is == RT_MUTEX_FULL_CHAINWALK.
+ *
+ * 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 bool rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter,
+                                          enum rtmutex_chainwalk chwalk)
+{
+        /*
+         * This is just a wrapper function for the following call,
+         * because debug_rt_mutex_detect_deadlock() smells like a magic
+         * debug feature and I wanted to keep the cond function in the
+         * main source file along with the comments instead of having
+         * two of the same in the headers.
+         */
+        return debug_rt_mutex_detect_deadlock(waiter, chwalk);
+}
+
+/*
  * Max number of times we'll walk the boosting chain:
  */
 int max_lock_depth = 1024;
@@ -381,7 +407,7 @@ static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
  *        goto again;
  */
 static int rt_mutex_adjust_prio_chain(struct task_struct *task,
-                                      int deadlock_detect,
+                                      enum rtmutex_chainwalk chwalk,
                                       struct rt_mutex *orig_lock,
                                       struct rt_mutex *next_lock,
                                       struct rt_mutex_waiter *orig_waiter,
@@ -389,12 +415,12 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
 {
         struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter;
         struct rt_mutex_waiter *prerequeue_top_waiter;
-        int detect_deadlock, ret = 0, depth = 0;
+        int ret = 0, depth = 0;
         struct rt_mutex *lock;
+        bool detect_deadlock;
         unsigned long flags;
 
-        detect_deadlock = debug_rt_mutex_detect_deadlock(orig_waiter,
-                                                         deadlock_detect);
+        detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);
 
         /*
          * The (de)boosting is a step by step approach with a lot of
@@ -520,7 +546,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
          * walk, we detected a deadlock.
          */
         if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
-                debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
+                debug_rt_mutex_deadlock(chwalk, orig_waiter, lock);
                 raw_spin_unlock(&lock->wait_lock);
                 ret = -EDEADLK;
                 goto out_unlock_pi;
@@ -784,7 +810,7 @@ takeit:
 static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
                                    struct rt_mutex_waiter *waiter,
                                    struct task_struct *task,
-                                   int detect_deadlock)
+                                   enum rtmutex_chainwalk chwalk)
 {
         struct task_struct *owner = rt_mutex_owner(lock);
         struct rt_mutex_waiter *top_waiter = waiter;
@@ -830,7 +856,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
                 __rt_mutex_adjust_prio(owner);
                 if (owner->pi_blocked_on)
                         chain_walk = 1;
-        } else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) {
+        } else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
                 chain_walk = 1;
         }
 
@@ -855,7 +881,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
 
         raw_spin_unlock(&lock->wait_lock);
 
-        res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock,
+        res = rt_mutex_adjust_prio_chain(owner, chwalk, lock,
                                          next_lock, waiter, task);
 
         raw_spin_lock(&lock->wait_lock);
@@ -960,7 +986,8 @@ static void remove_waiter(struct rt_mutex *lock,
 
         raw_spin_unlock(&lock->wait_lock);
 
-        rt_mutex_adjust_prio_chain(owner, 0, lock, next_lock, NULL, current);
+        rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
+                                   next_lock, NULL, current);
 
         raw_spin_lock(&lock->wait_lock);
 }
@@ -990,7 +1017,8 @@ void rt_mutex_adjust_pi(struct task_struct *task)
         /* gets dropped in rt_mutex_adjust_prio_chain()! */
         get_task_struct(task);
 
-        rt_mutex_adjust_prio_chain(task, 0, NULL, next_lock, NULL, task);
+        rt_mutex_adjust_prio_chain(task, RT_MUTEX_MIN_CHAINWALK, NULL,
+                                   next_lock, NULL, task);
 }
 
 /**
@@ -1068,7 +1096,7 @@ static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
 static int __sched
 rt_mutex_slowlock(struct rt_mutex *lock, int state,
                   struct hrtimer_sleeper *timeout,
-                  int detect_deadlock)
+                  enum rtmutex_chainwalk chwalk)
 {
         struct rt_mutex_waiter waiter;
         int ret = 0;
@@ -1094,7 +1122,7 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
                         timeout->task = NULL;
         }
 
-        ret = task_blocks_on_rt_mutex(lock, &waiter, current, detect_deadlock);
+        ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);
 
         if (likely(!ret))
                 ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
@@ -1103,7 +1131,7 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 
         if (unlikely(ret)) {
                 remove_waiter(lock, &waiter);
-                rt_mutex_handle_deadlock(ret, detect_deadlock, &waiter);
+                rt_mutex_handle_deadlock(ret, chwalk, &waiter);
         }
 
         /*
@@ -1230,27 +1258,29 @@ static inline int
 rt_mutex_fastlock(struct rt_mutex *lock, int state,
                   int (*slowfn)(struct rt_mutex *lock, int state,
                                 struct hrtimer_sleeper *timeout,
-                                int detect_deadlock))
+                                enum rtmutex_chainwalk chwalk))
 {
         if (likely(rt_mutex_cmpxchg(lock, NULL, current))) {
                 rt_mutex_deadlock_account_lock(lock, current);
                 return 0;
         } else
-                return slowfn(lock, state, NULL, 0);
+                return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK);
 }
 
 static inline int
 rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
-                        struct hrtimer_sleeper *timeout, int detect_deadlock,
+                        struct hrtimer_sleeper *timeout,
+                        enum rtmutex_chainwalk chwalk,
                         int (*slowfn)(struct rt_mutex *lock, int state,
                                       struct hrtimer_sleeper *timeout,
-                                      int detect_deadlock))
+                                      enum rtmutex_chainwalk chwalk))
 {
-        if (!detect_deadlock && likely(rt_mutex_cmpxchg(lock, NULL, current))) {
+        if (chwalk == RT_MUTEX_MIN_CHAINWALK &&
+            likely(rt_mutex_cmpxchg(lock, NULL, current))) {
                 rt_mutex_deadlock_account_lock(lock, current);
                 return 0;
         } else
-                return slowfn(lock, state, timeout, detect_deadlock);
+                return slowfn(lock, state, timeout, chwalk);
 }
 
 static inline int
@@ -1312,7 +1342,8 @@ int rt_mutex_timed_futex_lock(struct rt_mutex *lock,
 {
         might_sleep();
 
-        return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout, 1,
+        return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
+                                       RT_MUTEX_FULL_CHAINWALK,
                                        rt_mutex_slowlock);
 }
 
@@ -1334,7 +1365,8 @@ rt_mutex_timed_lock(struct rt_mutex *lock, struct hrtimer_sleeper *timeout)
 {
         might_sleep();
 
-        return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout, 0,
+        return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
+                                       RT_MUTEX_MIN_CHAINWALK,
                                        rt_mutex_slowlock);
 }
 EXPORT_SYMBOL_GPL(rt_mutex_timed_lock);
@@ -1463,7 +1495,8 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
         }
 
         /* We enforce deadlock detection for futexes */
-        ret = task_blocks_on_rt_mutex(lock, waiter, task, 1);
+        ret = task_blocks_on_rt_mutex(lock, waiter, task,
+                                      RT_MUTEX_FULL_CHAINWALK);
 
         if (ret && !rt_mutex_owner(lock)) {
                 /*
diff --git a/kernel/locking/rtmutex.h b/kernel/locking/rtmutex.h
index f6a1f3c133b1..c4060584c407 100644
--- a/kernel/locking/rtmutex.h
+++ b/kernel/locking/rtmutex.h
@@ -22,10 +22,15 @@
 #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 void rt_mutex_print_deadlock(struct rt_mutex_waiter *w)
 {
         WARN(1, "rtmutex deadlock detected\n");
 }
+
+static inline bool debug_rt_mutex_detect_deadlock(struct rt_mutex_waiter *w,
+                                                  enum rtmutex_chainwalk walk)
+{
+        return walk == RT_MUTEX_FULL_CHAINWALK;
+}
diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index cd3ec209d0c8..855212501407 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -102,6 +102,21 @@ static inline struct task_struct *rt_mutex_owner(struct rt_mutex *lock)
 }
 
 /*
+ * 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 rtmutex_chainwalk {
+        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);