Add rwsem_tryupgrade for 4.9.20-rt16 kernel

The RT rwsem implementation was changed to allow multiple readers
as of the 4.9.20-rt16 patch set.  This results in a build failure
because the existing implementation was forced to directly access
the rwsem structure which has changed.

While this could be accommodated by adding additional compatibility
code.  This patch resolves the build issue by simply assuming the
rwsem can never be upgraded.  This functionality is a performance
optimization and all callers must already handle this case.

Converting the last remaining use of __SPIN_LOCK_UNLOCKED to
spin_lock_init() was additionally required to get a clean build.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7589

include/spl/sys/rwlock.h

@@ -172,7 +172,7 @@ RW_LOCK_HELD(krwlock_t *rwp)
 }
 
 /*
- * The following functions must be a #define	and not static inline.
+ * The following functions must be a #define and not static inline.
  * This ensures that the native linux semaphore functions (down/up)
  * will be correctly located in the users code which is important
  * for the built in kernel lock analysis tools
@@ -188,10 +188,10 @@ RW_LOCK_HELD(krwlock_t *rwp)
 	spl_rw_set_type(rwp, type);					\
 })
 
-#define	rw_destroy(rwp)							\
-({									\
-	VERIFY(!RW_LOCK_HELD(rwp));					\
-})
+/*
+ * The Linux rwsem implementation does not require a matching destroy.
+ */
+#define	rw_destroy(rwp)		((void) 0)
 
 #define	rw_tryenter(rwp, rw)						\
 ({									\

module/spl/spl-rwlock.c

@@ -35,16 +35,24 @@
 static int
 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 {
-
+#if defined(READER_BIAS) && defined(WRITER_BIAS)
+	/*
+	 * After the 4.9.20-rt16 kernel the realtime patch series lifted the
+	 * single reader restriction.  While this could be accommodated by
+	 * adding additional compatibility code assume the rwsem can never
+	 * be upgraded.  All caller must already cleanly handle this case.
+	 */
+	return (0);
+#else
 	ASSERT((struct task_struct *)
 	    ((unsigned long)rwsem->lock.owner & ~RT_MUTEX_OWNER_MASKALL) ==
 	    current);
 
 	/*
-	 * Under the realtime patch series, rwsem is implemented as a
-	 * single mutex held by readers and writers alike. However,
-	 * this implementation would prevent a thread from taking a
-	 * read lock twice, as the mutex would already be locked on
+	 * Prior to 4.9.20-rt16 kernel the realtime patch series, rwsem is
+	 * implemented as a single mutex held by readers and writers alike.
+	 * However, this implementation would prevent a thread from taking
+	 * a read lock twice, as the mutex would already be locked on
 	 * the second attempt. Therefore the implementation allows a
 	 * single thread to take a rwsem as read lock multiple times
 	 * tracking that nesting as read_depth counter.
@@ -60,6 +68,7 @@ __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 		return (1);
 	}
 	return (0);
+#endif
 }
 #elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
 static int

module/spl/spl-vnode.c

@@ -744,7 +744,7 @@ vn_file_cache_destructor(void *buf, void *cdrarg)
 int
 spl_vn_init(void)
 {
-	vn_file_lock = __SPIN_LOCK_UNLOCKED(vn_file_lock);
+	spin_lock_init(&vn_file_lock);
 
 	vn_cache = kmem_cache_create("spl_vn_cache",
 	    sizeof (struct vnode), 64, vn_cache_constructor,