rt_mutex: add proxy lock routines

This patch is a prerequisite for futex requeue_pi. It basically splits
rt_mutex_slowlock() right down the middle, just before the first call
to schedule(). It further adds helper functions which make use of the
split and provide the rt-mutex preliminaries for futex requeue_pi.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 195 insertions, 53 deletions

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 69d9cb921ffa..fec77e7e0562 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -300,7 +300,8 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
  * assigned pending owner [which might not have taken the
  * lock yet]:
  */
-static inline int try_to_steal_lock(struct rt_mutex *lock)
+static inline int try_to_steal_lock(struct rt_mutex *lock,
+                                    struct task_struct *task)
 {
         struct task_struct *pendowner = rt_mutex_owner(lock);
         struct rt_mutex_waiter *next;
@@ -309,11 +310,11 @@ static inline int try_to_steal_lock(struct rt_mutex *lock)
         if (!rt_mutex_owner_pending(lock))
                 return 0;
 
-        if (pendowner == current)
+        if (pendowner == task)
                 return 1;
 
         spin_lock_irqsave(&pendowner->pi_lock, flags);
-        if (current->prio >= pendowner->prio) {
+        if (task->prio >= pendowner->prio) {
                 spin_unlock_irqrestore(&pendowner->pi_lock, flags);
                 return 0;
         }
@@ -338,21 +339,21 @@ static inline int try_to_steal_lock(struct rt_mutex *lock)
          * We are going to steal the lock and a waiter was
          * enqueued on the pending owners pi_waiters queue. So
          * we have to enqueue this waiter into
-         * current->pi_waiters list. This covers the case,
-         * where current is boosted because it holds another
+         * task->pi_waiters list. This covers the case,
+         * where task is boosted because it holds another
          * lock and gets unboosted because the booster is
          * interrupted, so we would delay a waiter with higher
-         * priority as current->normal_prio.
+         * priority as task->normal_prio.
          *
          * Note: in the rare case of a SCHED_OTHER task changing
          * its priority and thus stealing the lock, next->task
-         * might be current:
+         * might be task:
          */
-        if (likely(next->task != current)) {
-                spin_lock_irqsave(&current->pi_lock, flags);
-                plist_add(&next->pi_list_entry, &current->pi_waiters);
-                __rt_mutex_adjust_prio(current);
-                spin_unlock_irqrestore(&current->pi_lock, flags);
+        if (likely(next->task != task)) {
+                spin_lock_irqsave(&task->pi_lock, flags);
+                plist_add(&next->pi_list_entry, &task->pi_waiters);
+                __rt_mutex_adjust_prio(task);
+                spin_unlock_irqrestore(&task->pi_lock, flags);
         }
         return 1;
 }
@@ -389,7 +390,7 @@ static int try_to_take_rt_mutex(struct rt_mutex *lock)
          */
         mark_rt_mutex_waiters(lock);
 
-        if (rt_mutex_owner(lock) && !try_to_steal_lock(lock))
+        if (rt_mutex_owner(lock) && !try_to_steal_lock(lock, current))
                 return 0;
 
         /* We got the lock. */
@@ -411,6 +412,7 @@ static int try_to_take_rt_mutex(struct rt_mutex *lock)
  */
 static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
                                    struct rt_mutex_waiter *waiter,
+                                   struct task_struct *task,
                                    int detect_deadlock)
 {
         struct task_struct *owner = rt_mutex_owner(lock);
@@ -418,21 +420,21 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
         unsigned long flags;
         int chain_walk = 0, res;
 
-        spin_lock_irqsave(&current->pi_lock, flags);
-        __rt_mutex_adjust_prio(current);
-        waiter->task = current;
+        spin_lock_irqsave(&task->pi_lock, flags);
+        __rt_mutex_adjust_prio(task);
+        waiter->task = task;
         waiter->lock = lock;
-        plist_node_init(&waiter->list_entry, current->prio);
-        plist_node_init(&waiter->pi_list_entry, current->prio);
+        plist_node_init(&waiter->list_entry, task->prio);
+        plist_node_init(&waiter->pi_list_entry, task->prio);
 
         /* Get the top priority waiter on the lock */
         if (rt_mutex_has_waiters(lock))
                 top_waiter = rt_mutex_top_waiter(lock);
         plist_add(&waiter->list_entry, &lock->wait_list);
 
-        current->pi_blocked_on = waiter;
+        task->pi_blocked_on = waiter;
 
-        spin_unlock_irqrestore(&current->pi_lock, flags);
+        spin_unlock_irqrestore(&task->pi_lock, flags);
 
         if (waiter == rt_mutex_top_waiter(lock)) {
                 spin_lock_irqsave(&owner->pi_lock, flags);
@@ -460,7 +462,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
         spin_unlock(&lock->wait_lock);
 
         res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock, waiter,
-                                         current);
+                                         task);
 
         spin_lock(&lock->wait_lock);
 
@@ -605,37 +607,25 @@ void rt_mutex_adjust_pi(struct task_struct *task)
         rt_mutex_adjust_prio_chain(task, 0, NULL, NULL, task);
 }
 
-/*
- * Slow path lock function:
+/**
+ * __rt_mutex_slowlock() - Perform the wait-wake-try-to-take loop
+ * @lock:                the rt_mutex to take
+ * @state:               the state the task should block in (TASK_INTERRUPTIBLE
+ *                       or TASK_UNINTERRUPTIBLE)
+ * @timeout:             the pre-initialized and started timer, or NULL for none
+ * @waiter:              the pre-initialized rt_mutex_waiter
+ * @detect_deadlock:     passed to task_blocks_on_rt_mutex
+ *
+ * lock->wait_lock must be held by the caller.
  */
 static int __sched
-rt_mutex_slowlock(struct rt_mutex *lock, int state,
-                  struct hrtimer_sleeper *timeout,
-                  int detect_deadlock)
+__rt_mutex_slowlock(struct rt_mutex *lock, int state,
+                    struct hrtimer_sleeper *timeout,
+                    struct rt_mutex_waiter *waiter,
+                    int detect_deadlock)
 {
-        struct rt_mutex_waiter waiter;
         int ret = 0;
 
-        debug_rt_mutex_init_waiter(&waiter);
-        waiter.task = NULL;
-
-        spin_lock(&lock->wait_lock);
-
-        /* Try to acquire the lock again: */
-        if (try_to_take_rt_mutex(lock)) {
-                spin_unlock(&lock->wait_lock);
-                return 0;
-        }
-
-        set_current_state(state);
-
-        /* Setup the timer, when timeout != NULL */
-        if (unlikely(timeout)) {
-                hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
-                if (!hrtimer_active(&timeout->timer))
-                        timeout->task = NULL;
-        }
-
         for (;;) {
                 /* Try to acquire the lock: */
                 if (try_to_take_rt_mutex(lock))
@@ -656,19 +646,19 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
                 }
 
                 /*
-                 * waiter.task is NULL the first time we come here and
+                 * waiter->task is NULL the first time we come here and
                  * when we have been woken up by the previous owner
                  * but the lock got stolen by a higher prio task.
                  */
-                if (!waiter.task) {
-                        ret = task_blocks_on_rt_mutex(lock, &waiter,
+                if (!waiter->task) {
+                        ret = task_blocks_on_rt_mutex(lock, waiter, current,
                                                       detect_deadlock);
                         /*
                          * If we got woken up by the owner then start loop
                          * all over without going into schedule to try
                          * to get the lock now:
                          */
-                        if (unlikely(!waiter.task)) {
+                        if (unlikely(!waiter->task)) {
                                 /*
                                  * Reset the return value. We might
                                  * have returned with -EDEADLK and the
@@ -684,15 +674,52 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 
                 spin_unlock(&lock->wait_lock);
 
-                debug_rt_mutex_print_deadlock(&waiter);
+                debug_rt_mutex_print_deadlock(waiter);
 
-                if (waiter.task)
+                if (waiter->task)
                         schedule_rt_mutex(lock);
 
                 spin_lock(&lock->wait_lock);
                 set_current_state(state);
         }
 
+        return ret;
+}
+
+/*
+ * Slow path lock function:
+ */
+static int __sched
+rt_mutex_slowlock(struct rt_mutex *lock, int state,
+                  struct hrtimer_sleeper *timeout,
+                  int detect_deadlock)
+{
+        struct rt_mutex_waiter waiter;
+        int ret = 0;
+
+        debug_rt_mutex_init_waiter(&waiter);
+        waiter.task = NULL;
+
+        spin_lock(&lock->wait_lock);
+
+        /* Try to acquire the lock again: */
+        if (try_to_take_rt_mutex(lock)) {
+                spin_unlock(&lock->wait_lock);
+                return 0;
+        }
+
+        set_current_state(state);
+
+        /* Setup the timer, when timeout != NULL */
+        if (unlikely(timeout)) {
+                hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
+                if (!hrtimer_active(&timeout->timer))
+                        timeout->task = NULL;
+        }
+
+        ret = __rt_mutex_slowlock(lock, state, timeout, &waiter,
+                                  detect_deadlock);
+
         set_current_state(TASK_RUNNING);
 
         if (unlikely(waiter.task))
@@ -986,6 +1013,59 @@ void rt_mutex_proxy_unlock(struct rt_mutex *lock,
 }
 
 /**
+ * rt_mutex_start_proxy_lock() - Start lock acquisition for another task
+ * @lock:               the rt_mutex to take
+ * @waiter:             the pre-initialized rt_mutex_waiter
+ * @task:               the task to prepare
+ * @detect_deadlock:    perform deadlock detection (1) or not (0)
+ *
+ * Returns:
+ *  0 - task blocked on lock
+ *  1 - acquired the lock for task, caller should wake it up
+ * <0 - error
+ *
+ * Special API call for FUTEX_REQUEUE_PI support.
+ */
+int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
+                              struct rt_mutex_waiter *waiter,
+                              struct task_struct *task, int detect_deadlock)
+{
+        int ret;
+
+        spin_lock(&lock->wait_lock);
+
+        mark_rt_mutex_waiters(lock);
+
+        if (!rt_mutex_owner(lock) || try_to_steal_lock(lock, task)) {
+                /* We got the lock for task. */
+                debug_rt_mutex_lock(lock);
+
+                rt_mutex_set_owner(lock, task, 0);
+
+                rt_mutex_deadlock_account_lock(lock, task);
+                return 1;
+        }
+
+        ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);
+
+
+        if (ret && !waiter->task) {
+                /*
+                 * Reset the return value. We might have
+                 * returned with -EDEADLK and the owner
+                 * released the lock while we were walking the
+                 * pi chain.  Let the waiter sort it out.
+                 */
+                ret = 0;
+        }
+        spin_unlock(&lock->wait_lock);
+
+        debug_rt_mutex_print_deadlock(waiter);
+
+        return ret;
+}
+
+/**
  * rt_mutex_next_owner - return the next owner of the lock
  *
  * @lock: the rt lock query
@@ -1004,3 +1084,57 @@ struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
 
         return rt_mutex_top_waiter(lock)->task;
 }
+
+/**
+ * rt_mutex_finish_proxy_lock() - Complete lock acquisition
+ * @lock:               the rt_mutex we were woken on
+ * @to:                 the timeout, null if none. hrtimer should already have
+ *                      been started.
+ * @waiter:             the pre-initialized rt_mutex_waiter
+ * @detect_deadlock:    perform deadlock detection (1) or not (0)
+ *
+ * Complete the lock acquisition started our behalf by another thread.
+ *
+ * Returns:
+ *  0 - success
+ * <0 - error, one of -EINTR, -ETIMEDOUT, or -EDEADLK
+ *
+ * Special API call for PI-futex requeue support
+ */
+int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
+                               struct hrtimer_sleeper *to,
+                               struct rt_mutex_waiter *waiter,
+                               int detect_deadlock)
+{
+        int ret;
+
+        spin_lock(&lock->wait_lock);
+
+        set_current_state(TASK_INTERRUPTIBLE);
+
+        ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter,
+                                  detect_deadlock);
+
+        set_current_state(TASK_RUNNING);
+
+        if (unlikely(waiter->task))
+                remove_waiter(lock, waiter);
+
+        /*
+         * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
+         * have to fix that up.
+         */
+        fixup_rt_mutex_waiters(lock);
+
+        spin_unlock(&lock->wait_lock);
+
+        /*
+         * Readjust priority, when we did not get the lock. We might have been
+         * the pending owner and boosted. Since we did not take the lock, the
+         * PI boost has to go.
+         */
+        if (unlikely(ret))
+                rt_mutex_adjust_prio(current);
+
+        return ret;
+}
diff --git a/kernel/rtmutex_common.h b/kernel/rtmutex_common.h
index e124bf5800ea..97a2f81866af 100644
--- a/kernel/rtmutex_common.h
+++ b/kernel/rtmutex_common.h
@@ -120,6 +120,14 @@ extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
                                        struct task_struct *proxy_owner);
 extern void rt_mutex_proxy_unlock(struct rt_mutex *lock,
                                   struct task_struct *proxy_owner);
+extern int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
+                                     struct rt_mutex_waiter *waiter,
+                                     struct task_struct *task,
+                                     int detect_deadlock);
+extern int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
+                                      struct hrtimer_sleeper *to,
+                                      struct rt_mutex_waiter *waiter,
+                                      int detect_deadlock);
 
 #ifdef CONFIG_DEBUG_RT_MUTEXES
 # include "rtmutex-debug.h"