+/*
+ * The reason for the spinlock:
+ *
+ * The Linux mutex is designed with a fast-path/slow-path design such that it
+ * does not guarantee serialization upon itself, allowing a race where latter
+ * acquirers finish mutex_unlock before former ones.
+ *
+ * The race renders it unsafe to be used for serializing the freeing of an
+ * object in which the mutex is embedded, where the latter acquirer could go
+ * on to free the object while the former one is still doing mutex_unlock and
+ * causing memory corruption.
+ *
+ * However, there are many places in ZFS where the mutex is used for
+ * serializing object freeing, and the code is shared among other OSes without
+ * this issue. Thus, we need the spinlock to force the serialization on
+ * mutex_exit().
+ *
+ * See http://lwn.net/Articles/575477/ for the information about the race.
+ */
+#define mutex_exit(mp) \
+({ \
+ spin_lock(&(mp)->m_lock); \
+ mutex_unlock(&(mp)->m); \
+ spin_unlock(&(mp)->m_lock); \
+})