itimers: Handle relative timers with CONFIG_TIME_LOW_RES proper

[pandora-kernel.git] / kernel / rtmutex.c

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index a72b99f..1928f3d 100644 (file)
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -81,6 +81,47 @@ static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
                owner = *p;
        } while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
 }
+
+/*
+ * Safe fastpath aware unlock:
+ * 1) Clear the waiters bit
+ * 2) Drop lock->wait_lock
+ * 3) Try to unlock the lock with cmpxchg
+ */
+static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
+       __releases(lock->wait_lock)
+{
+       struct task_struct *owner = rt_mutex_owner(lock);
+
+       clear_rt_mutex_waiters(lock);
+       raw_spin_unlock(&lock->wait_lock);
+       /*
+        * If a new waiter comes in between the unlock and the cmpxchg
+        * we have two situations:
+        *
+        * unlock(wait_lock);
+        *                                      lock(wait_lock);
+        * cmpxchg(p, owner, 0) == owner
+        *                                      mark_rt_mutex_waiters(lock);
+        *                                      acquire(lock);
+        * or:
+        *
+        * unlock(wait_lock);
+        *                                      lock(wait_lock);
+        *                                      mark_rt_mutex_waiters(lock);
+        *
+        * cmpxchg(p, owner, 0) != owner
+        *                                      enqueue_waiter();
+        *                                      unlock(wait_lock);
+        * lock(wait_lock);
+        * wake waiter();
+        * unlock(wait_lock);
+        *                                      lock(wait_lock);
+        *                                      acquire(lock);
+        */
+       return rt_mutex_cmpxchg(lock, owner, NULL);
+}
+
 #else
 # define rt_mutex_cmpxchg(l,c,n)       (0)
 static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
@@ -88,6 +129,17 @@ static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
        lock->owner = (struct task_struct *)
                        ((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
 }
+
+/*
+ * Simple slow path only version: lock->owner is protected by lock->wait_lock.
+ */
+static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
+       __releases(lock->wait_lock)
+{
+       lock->owner = NULL;
+       raw_spin_unlock(&lock->wait_lock);
+       return true;
+}
 #endif
 
 /*
@@ -535,7 +587,8 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
 /*
  * Wake up the next waiter on the lock.
  *
- * Remove the top waiter from the current tasks waiter list and wake it up.
+ * Remove the top waiter from the current tasks pi waiter list and
+ * wake it up.
  *
  * Called with lock->wait_lock held.
  */
@@ -556,10 +609,23 @@ static void wakeup_next_waiter(struct rt_mutex *lock)
         */
        plist_del(&waiter->pi_list_entry, &current->pi_waiters);
 
-       rt_mutex_set_owner(lock, NULL);
+       /*
+        * As we are waking up the top waiter, and the waiter stays
+        * queued on the lock until it gets the lock, this lock
+        * obviously has waiters. Just set the bit here and this has
+        * the added benefit of forcing all new tasks into the
+        * slow path making sure no task of lower priority than
+        * the top waiter can steal this lock.
+        */
+       lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
 
        raw_spin_unlock_irqrestore(&current->pi_lock, flags);
 
+       /*
+        * It's safe to dereference waiter as it cannot go away as
+        * long as we hold lock->wait_lock. The waiter task needs to
+        * acquire it in order to dequeue the waiter.
+        */
        wake_up_process(waiter->task);
 }
 
@@ -820,12 +886,49 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
 
        rt_mutex_deadlock_account_unlock(current);
 
-       if (!rt_mutex_has_waiters(lock)) {
-               lock->owner = NULL;
-               raw_spin_unlock(&lock->wait_lock);
-               return;
+       /*
+        * We must be careful here if the fast path is enabled. If we
+        * have no waiters queued we cannot set owner to NULL here
+        * because of:
+        *
+        * foo->lock->owner = NULL;
+        *                      rtmutex_lock(foo->lock);   <- fast path
+        *                      free = atomic_dec_and_test(foo->refcnt);
+        *                      rtmutex_unlock(foo->lock); <- fast path
+        *                      if (free)
+        *                              kfree(foo);
+        * raw_spin_unlock(foo->lock->wait_lock);
+        *
+        * So for the fastpath enabled kernel:
+        *
+        * Nothing can set the waiters bit as long as we hold
+        * lock->wait_lock. So we do the following sequence:
+        *
+        *      owner = rt_mutex_owner(lock);
+        *      clear_rt_mutex_waiters(lock);
+        *      raw_spin_unlock(&lock->wait_lock);
+        *      if (cmpxchg(&lock->owner, owner, 0) == owner)
+        *              return;
+        *      goto retry;
+        *
+        * The fastpath disabled variant is simple as all access to
+        * lock->owner is serialized by lock->wait_lock:
+        *
+        *      lock->owner = NULL;
+        *      raw_spin_unlock(&lock->wait_lock);
+        */
+       while (!rt_mutex_has_waiters(lock)) {
+               /* Drops lock->wait_lock ! */
+               if (unlock_rt_mutex_safe(lock) == true)
+                       return;
+               /* Relock the rtmutex and try again */
+               raw_spin_lock(&lock->wait_lock);
        }
 
+       /*
+        * The wakeup next waiter path does not suffer from the above
+        * race. See the comments there.
+        */
        wakeup_next_waiter(lock);
 
        raw_spin_unlock(&lock->wait_lock);