[RFC PATCH 04/10] xen: add reference counter support

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

We can use reference counter to ease up object lifetime management.
This patch adds very basic support for reference counters. refcnt
should be used in the following way:

1. Protected structure should have refcnt_t field

2. This field should be initialized with refcnt_init() during object
construction.

3. If code holds a valid pointer to a structure/object it can increase
refcount with refcnt_get(). No additional locking is required.

4. Code should call refcnt_put() before dropping pointer to a
protected structure. `destructor` is a call back function that should
destruct object and free all resources, including structure protected
itself. Destructor will be called if reference counter reaches zero.

5. If code does not hold a valid pointer to a protected structure it
should use other locking mechanism to obtain a pointer. For example,
it should lock a list that hold protected objects.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
---
 xen/include/xen/refcnt.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)
 create mode 100644 xen/include/xen/refcnt.h

diff --git a/xen/include/xen/refcnt.h b/xen/include/xen/refcnt.h
new file mode 100644
index 0000000000..7f5395a21c
--- /dev/null
+++ b/xen/include/xen/refcnt.h
@@ -0,0 +1,28 @@
+#ifndef __XEN_REFCNT_H__
+#define __XEN_REFCNT_H__
+
+#include <asm/atomic.h>
+
+typedef atomic_t refcnt_t;
+
+static inline void refcnt_init(refcnt_t *refcnt)
+{
+       atomic_set(refcnt, 1);
+}
+
+static inline void refcnt_get(refcnt_t *refcnt)
+{
+#ifndef NDEBUG
+       ASSERT(atomic_add_unless(refcnt, 1, 0) > 0);
+#else
+       atomic_add_unless(refcnt, 1, 0);
+#endif
+}
+
+static inline void refcnt_put(refcnt_t *refcnt, void (*destructor)(refcnt_t 
*refcnt))
+{
+       if ( atomic_dec_and_test(refcnt) )
+               destructor(refcnt);
+}
+
+#endif
-- 
2.36.1