Add additional examples in Documentation/spinlocks.txt

Checkpatch will throw an error if code doesn't use the correct initializers
for static spinlocks:

ERROR: Use of SPIN_LOCK_UNLOCKED is deprecated: see Documentation/spinlocks.txt

This is fine, but Documentation/spinlocks.txt isn't very clear on how to
_use_ the new initializers for static variables. To save people time in the
future, I added two small examples of how to fix old-style static
initializers to be more lockdep friendly.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 22 insertions, 0 deletions

diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt
index 471e7538977..619699dde59 100644
--- a/Documentation/spinlocks.txt
+++ b/Documentation/spinlocks.txt
@@ -5,6 +5,28 @@ Please use DEFINE_SPINLOCK()/DEFINE_RWLOCK() or
 __SPIN_LOCK_UNLOCKED()/__RW_LOCK_UNLOCKED() as appropriate for static
 initialization.
 
+Most of the time, you can simply turn:
+
+        static spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
+
+into:
+
+        static DEFINE_SPINLOCK(xxx_lock);
+
+Static structure member variables go from:
+
+        struct foo bar {
+                .lock   =       SPIN_LOCK_UNLOCKED;
+        };
+
+to:
+
+        struct foo bar {
+                .lock   =       __SPIN_LOCK_UNLOCKED(bar.lock);
+        };
+
+Declaration of static rw_locks undergo a similar transformation.
+
 Dynamic initialization, when necessary, may be performed as
 demonstrated below.