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authorPaul Menage <menage@google.com>2009-01-07 18:08:38 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2009-01-08 08:31:10 -0800
commite7c5ec9193d32b9559a3bb8893ceedbda85201ff (patch)
treefd7505c03e2c7525a110a702907c0604c233e2d9 /include
parent2cb378c862777d050c20db903b119a029845fdcb (diff)
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>
Diffstat (limited to 'include')
-rw-r--r--include/linux/cgroup.h38
1 files changed, 32 insertions, 6 deletions
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ce1c1f34c30..e267e62827b 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -52,9 +52,9 @@ struct cgroup_subsys_state {
* 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 @@ struct cgroup_subsys_state {
/* 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 @@ static inline void css_get(struct cgroup_subsys_state *css)
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);