cgroups: add css_tryget()
Add css_tryget(), that obtains a counted reference on a CSS. It is used
in situations where the caller has a "weak" reference to the CSS, i.e.
one that does not protect the cgroup from removal via a reference count,
but would instead be cleaned up by a destroy() callback.
css_tryget() will return true on success, or false if the cgroup is being
removed.
This is similar to Kamezawa Hiroyuki's patch from a week or two ago, but
with the difference that in the event of css_tryget() racing with a
cgroup_rmdir(), css_tryget() will only return false if the cgroup really
does get removed.
This implementation is done by biasing css->refcnt, so that a refcnt of 1
means "releasable" and 0 means "released or releasing". In the event of a
race, css_tryget() distinguishes between "released" and "releasing" by
checking for the CSS_REMOVED flag in css->flags.
Signed-off-by: Paul Menage <menage@google.com>
Tested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ce1c1f3..e267e628 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -52,9 +52,9 @@
* hierarchy structure */
struct cgroup *cgroup;
- /* State maintained by the cgroup system to allow
- * subsystems to be "busy". Should be accessed via css_get()
- * and css_put() */
+ /* State maintained by the cgroup system to allow subsystems
+ * to be "busy". Should be accessed via css_get(),
+ * css_tryget() and and css_put(). */
atomic_t refcnt;
@@ -64,11 +64,14 @@
/* bits in struct cgroup_subsys_state flags field */
enum {
CSS_ROOT, /* This CSS is the root of the subsystem */
+ CSS_REMOVED, /* This CSS is dead */
};
/*
- * Call css_get() to hold a reference on the cgroup;
- *
+ * Call css_get() to hold a reference on the css; it can be used
+ * for a reference obtained via:
+ * - an existing ref-counted reference to the css
+ * - task->cgroups for a locked task
*/
static inline void css_get(struct cgroup_subsys_state *css)
@@ -77,9 +80,32 @@
if (!test_bit(CSS_ROOT, &css->flags))
atomic_inc(&css->refcnt);
}
+
+static inline bool css_is_removed(struct cgroup_subsys_state *css)
+{
+ return test_bit(CSS_REMOVED, &css->flags);
+}
+
+/*
+ * Call css_tryget() to take a reference on a css if your existing
+ * (known-valid) reference isn't already ref-counted. Returns false if
+ * the css has been destroyed.
+ */
+
+static inline bool css_tryget(struct cgroup_subsys_state *css)
+{
+ if (test_bit(CSS_ROOT, &css->flags))
+ return true;
+ while (!atomic_inc_not_zero(&css->refcnt)) {
+ if (test_bit(CSS_REMOVED, &css->flags))
+ return false;
+ }
+ return true;
+}
+
/*
* css_put() should be called to release a reference taken by
- * css_get()
+ * css_get() or css_tryget()
*/
extern void __css_put(struct cgroup_subsys_state *css);