Linux 4.8 compat: rw_semaphore atomic_long_t count

For non-rwsem-spinlocks the "count" member was changed from a
"long" to "atomic_long_t" type.  A configure check has been
added to detect this change along with new versions of the
_rwsem_tryupgrade() function and RWSEM_COUNT() macro.  See
torvalds/linux/commit/8ee62b18 for complete details.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #563

config/spl-build.m4

@@ -40,6 +40,7 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_SHRINK_CONTROL_STRUCT
 	SPL_AC_RWSEM_SPINLOCK_IS_RAW
 	SPL_AC_RWSEM_ACTIVITY
+	SPL_AC_RWSEM_ATOMIC_LONG_COUNT
 	SPL_AC_SCHED_RT_HEADER
 	SPL_AC_2ARGS_VFS_GETATTR
 	SPL_AC_USLEEP_RANGE
@@ -1341,6 +1342,31 @@ AC_DEFUN([SPL_AC_RWSEM_ACTIVITY], [
 	EXTRA_KCFLAGS="$tmp_flags"
 ])
 
+dnl #
+dnl # 4.8 API Change
+dnl #
+dnl # rwsem "->count" changed to atomic_long_t type
+dnl #
+AC_DEFUN([SPL_AC_RWSEM_ATOMIC_LONG_COUNT], [
+	AC_MSG_CHECKING(
+	[whether struct rw_semaphore has atomic_long_t member count])
+	tmp_flags="$EXTRA_KCFLAGS"
+	EXTRA_KCFLAGS="-Werror"
+	SPL_LINUX_TRY_COMPILE([
+		#include <linux/rwsem.h>
+	],[
+		DECLARE_RWSEM(dummy_semaphore);
+		(void) atomic_long_read(&dummy_semaphore.count);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_RWSEM_ATOMIC_LONG_COUNT, 1,
+		[struct rw_semaphore has atomic_long_t member count])
+	],[
+		AC_MSG_RESULT(no)
+	])
+	EXTRA_KCFLAGS="$tmp_flags"
+])
+
 dnl #
 dnl # 3.9 API change,
 dnl # Moved things from linux/sched.h to linux/sched/rt.h

include/linux/rwsem_compat.h

@@ -35,9 +35,12 @@
 #define	SPL_RWSEM_SINGLE_WRITER_VALUE	(RWSEM_ACTIVE_WRITE_BIAS)
 #endif
 
-/* Linux 3.16 change activity to count for rwsem-spinlock */
-#ifdef HAVE_RWSEM_ACTIVITY
+/* Linux 3.16 changed activity to count for rwsem-spinlock */
+#if defined(HAVE_RWSEM_ACTIVITY)
 #define	RWSEM_COUNT(sem)	sem->activity
+/* Linux 4.8 changed count to an atomic_long_t for !rwsem-spinlock */
+#elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
+#define	RWSEM_COUNT(sem)	atomic_long_read(&(sem)->count)
 #else
 #define	RWSEM_COUNT(sem)	sem->count
 #endif

module/spl/spl-rwlock.c

@@ -32,7 +32,7 @@
 
 #define DEBUG_SUBSYSTEM S_RWLOCK
 
-#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
+#if defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
 static int
 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 {
@@ -47,6 +47,15 @@ __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 	spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
 	return (ret);
 }
+#elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
+static int
+__rwsem_tryupgrade(struct rw_semaphore *rwsem)
+{
+	long val;
+	val = atomic_long_cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
+	    SPL_RWSEM_SINGLE_WRITER_VALUE);
+	return (val == SPL_RWSEM_SINGLE_READER_VALUE);
+}
 #else
 static int
 __rwsem_tryupgrade(struct rw_semaphore *rwsem)