[PATCH 7/7] rcu: support headless variant in the kvfree_rcu()

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Make it possible to pass one or two arguments to the
kvfree_rcu() macro what corresponds to either headless
case or not, so it becomes a bit versatile.

As a result we obtain two ways of using that macro,
below are two examples:

a) kvfree_rcu(ptr, rhf);
    struct X {
        struct rcu_head rhf;
        unsigned char data[100];
    };

    void *ptr = kvmalloc(sizeof(struct X), GFP_KERNEL);
    if (ptr)
        kvfree_rcu(ptr, rhf);

b) kvfree_rcu(ptr);
    void *ptr = kvmalloc(some_bytes, GFP_KERNEL);
    if (ptr)
        kvfree_rcu(ptr);

Last one, we name it headless variant, only needs one
argument, means it does not require any rcu_head to be
present within the type of ptr.

There is a restriction the (b) context has to fall into
might_sleep() annotation. To check that, please activate
the CONFIG_DEBUG_ATOMIC_SLEEP option in your kernel.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 include/linux/rcupdate.h | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index efd3dc13c36d..51ebf2461d51 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -844,12 +844,42 @@ do {									\
 
 /**
  * kvfree_rcu() - kvfree an object after a grace period.
- * @ptr:	pointer to kvfree
- * @rhf:	the name of the struct rcu_head within the type of @ptr.
  *
- * Same as kfree_rcu(), just simple alias.
+ * This macro consists of one or two arguments and it is
+ * based on whether an object is head-less or not. If it
+ * has a head then a semantic stays the same as it used
+ * to be before:
+ *
+ *     kvfree_rcu(ptr, rhf);
+ *
+ * where @ptr is a pointer to kvfree(), @rhf is the name
+ * of the rcu_head structure within the type of @ptr.
+ *
+ * When it comes to head-less variant, only one argument
+ * is passed and that is just a pointer which has to be
+ * freed after a grace period. Therefore the semantic is
+ *
+ *     kvfree_rcu(ptr);
+ *
+ * where @ptr is a pointer to kvfree().
+ *
+ * Please note, head-less way of freeing is permitted to
+ * use from a context that has to follow might_sleep()
+ * annotation. Otherwise, please switch and embed the
+ * rcu_head structure within the type of @ptr.
  */
-#define kvfree_rcu(ptr, rhf) kfree_rcu(ptr, rhf)
+#define kvfree_rcu(...) KVFREE_GET_MACRO(__VA_ARGS__,		\
+	kvfree_rcu_arg_2, kvfree_rcu_arg_1)(__VA_ARGS__)
+
+#define KVFREE_GET_MACRO(_1, _2, NAME, ...) NAME
+#define kvfree_rcu_arg_2(ptr, rhf) kfree_rcu(ptr, rhf)
+#define kvfree_rcu_arg_1(ptr)					\
+do {								\
+	typeof(ptr) ___p = (ptr);				\
+								\
+	if (___p)						\
+		kvfree_call_rcu(NULL, (rcu_callback_t) (___p));	\
+} while (0)
 
 /*
  * Place this after a lock-acquisition primitive to guarantee that
-- 
2.20.1