core: add "gf_ref_t" for common refcounting structures

Checks for compiler supported atomic operations comes from client_t.h.

An example usage of this change can be found in adding reference
counting to "struct auth_cache_entry" in http://review.gluster.org/11023

Basic usage looks like this:

   #include "refcount.h"

   struct my_struct {
       GF_REF_DECL;
       ... /* more members */
   }

   void my_destructor (void *data)
   {
       struct my_struct *my_ptr = (struct my_struct *) data;

       ... /* do some more cleanups */
       GF_FREE (my_ptr);
   }

   void init_ptr (struct parent *parent)
   {
       struct my_struct *my_ptr = malloc (sizeof (struct my_struct));

       GF_REF_INIT (my_ptr, my_destructor); /* refcount is set to 1 */

       ... /* my_ptr probably gets added to some parent structure */
       parent_add_ptr (parent, my_ptr);
   }

   void do_something (struct parent *parent)
   {
       struct my_struct *my_ptr = NULL;

       /* likely need to lock parent, depends on its access pattern */
       my_ptr = parent_remove_first_ptr (parent);
       /* unlock parent */

       ... /* do something */

       GF_REF_PUT (my_ptr); /* calls my_destructor on refcount = 0 */
   }

URL: http://thread.gmane.org/gmane.comp.file-systems.gluster.devel/11202
Change-Id: Idb98a5861a44c31676108ed8876db12c320912ef
BUG: 1228157
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/11022
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Tested-by: NetBSD Build System <jenkins@build.gluster.org>

1 files changed, 112 insertions, 0 deletions

diff --git a/libglusterfs/src/refcount.c b/libglusterfs/src/refcount.c
new file mode 100644
index 00000000000..96edc10982a
--- /dev/null
+++ b/libglusterfs/src/refcount.c
@@ -0,0 +1,112 @@
+/*
+  Copyright (c) 2015 Red Hat, Inc. <http://www.redhat.com>
+  This file is part of GlusterFS.
+
+  This file is licensed to you under your choice of the GNU Lesser
+  General Public License, version 3 or any later version (LGPLv3 or
+  later), or the GNU General Public License, version 2 (GPLv2), in all
+  cases as published by the Free Software Foundation.
+*/
+
+#include "common-utils.h"
+#include "refcount.h"
+
+#ifndef REFCOUNT_NEEDS_LOCK
+
+unsigned int
+_gf_ref_get (gf_ref_t *ref)
+{
+        unsigned int cnt = __sync_fetch_and_add (&ref->cnt, 1);
+
+        /* if cnt == 0, we're in a fatal position, the object will be free'd
+         *
+         * There is a race when two threads do a _gf_ref_get(). Only one of
+         * them may get a 0 returned. That is acceptible, because one
+         * _gf_ref_get() returning 0 should be handled as a fatal problem and
+         * when correct usage/locking is used, it should never happen.
+         */
+        GF_ASSERT (cnt != 0);
+
+        return cnt;
+}
+
+unsigned int
+_gf_ref_put (gf_ref_t *ref)
+{
+        unsigned int cnt = __sync_fetch_and_sub (&ref->cnt, 1);
+
+        /* if cnt == 1, the last user just did a _gf_ref_put()
+         *
+         * When cnt == 0, one _gf_ref_put() was done too much and there has
+         * been a thread using the refcounted structure when it was not
+         * supposed to.
+         */
+        GF_ASSERT (cnt != 0);
+
+        if (cnt == 1 && ref->release) {
+                ref->release (ref->data);
+                /* set return value to 0 to inform the caller correctly */
+                cnt = 0;
+        }
+
+        return cnt;
+}
+
+#else
+
+unsigned int
+_gf_ref_get (gf_ref_t *ref)
+{
+        unsigned int cnt = 0;
+
+        LOCK (&ref->lk);
+        {
+                /* never can be 0, should have been free'd */
+                if (ref->cnt > 0)
+                        cnt = ++ref->cnt;
+                else
+                        GF_ASSERT (ref->cnt > 0);
+        }
+        UNLOCK (&ref->lk);
+
+        return cnt;
+}
+
+unsigned int
+_gf_ref_put (gf_ref_t *ref)
+{
+        unsigned int cnt = 0;
+        int release = 0;
+
+        LOCK (&ref->lk);
+        {
+                if (ref->cnt != 0) {
+                        cnt = --ref->cnt;
+                        /* call release() only when cnt == 0 */
+                        release = (cnt == 0);
+                } else
+                        GF_ASSERT (ref->cnt != 0);
+        }
+        UNLOCK (&ref->lk);
+
+        if (release && ref->release)
+                ref->release (ref->data);
+
+        return cnt;
+}
+
+#endif /* REFCOUNT_NEEDS_LOCK */
+
+
+void
+_gf_ref_init (gf_ref_t *ref, gf_ref_release_t release, void *data)
+{
+        GF_ASSERT (ref);
+
+#ifdef REFCOUNT_NEEDS_LOCK
+        LOCK_INIT (&ref->lk);
+#endif
+        ref->cnt = 1;
+        ref->release = release;
+        ref->data = data;
+}