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-rw-r--r--kernel/locking/rtmutex.c68
1 files changed, 66 insertions, 2 deletions
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 1ec0f48962b3..2c49d76f96c3 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -65,8 +65,72 @@ static inline void clear_rt_mutex_waiters(struct rt_mutex *lock)
static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
{
- if (!rt_mutex_has_waiters(lock))
- clear_rt_mutex_waiters(lock);
+ unsigned long owner, *p = (unsigned long *) &lock->owner;
+
+ if (rt_mutex_has_waiters(lock))
+ return;
+
+ /*
+ * The rbtree has no waiters enqueued, now make sure that the
+ * lock->owner still has the waiters bit set, otherwise the
+ * following can happen:
+ *
+ * CPU 0 CPU 1 CPU2
+ * l->owner=T1
+ * rt_mutex_lock(l)
+ * lock(l->lock)
+ * l->owner = T1 | HAS_WAITERS;
+ * enqueue(T2)
+ * boost()
+ * unlock(l->lock)
+ * block()
+ *
+ * rt_mutex_lock(l)
+ * lock(l->lock)
+ * l->owner = T1 | HAS_WAITERS;
+ * enqueue(T3)
+ * boost()
+ * unlock(l->lock)
+ * block()
+ * signal(->T2) signal(->T3)
+ * lock(l->lock)
+ * dequeue(T2)
+ * deboost()
+ * unlock(l->lock)
+ * lock(l->lock)
+ * dequeue(T3)
+ * ==> wait list is empty
+ * deboost()
+ * unlock(l->lock)
+ * lock(l->lock)
+ * fixup_rt_mutex_waiters()
+ * if (wait_list_empty(l) {
+ * l->owner = owner
+ * owner = l->owner & ~HAS_WAITERS;
+ * ==> l->owner = T1
+ * }
+ * lock(l->lock)
+ * rt_mutex_unlock(l) fixup_rt_mutex_waiters()
+ * if (wait_list_empty(l) {
+ * owner = l->owner & ~HAS_WAITERS;
+ * cmpxchg(l->owner, T1, NULL)
+ * ===> Success (l->owner = NULL)
+ *
+ * l->owner = owner
+ * ==> l->owner = T1
+ * }
+ *
+ * With the check for the waiter bit in place T3 on CPU2 will not
+ * overwrite. All tasks fiddling with the waiters bit are
+ * serialized by l->lock, so nothing else can modify the waiters
+ * bit. If the bit is set then nothing can change l->owner either
+ * so the simple RMW is safe. The cmpxchg() will simply fail if it
+ * happens in the middle of the RMW because the waiters bit is
+ * still set.
+ */
+ owner = READ_ONCE(*p);
+ if (owner & RT_MUTEX_HAS_WAITERS)
+ WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
}
/*