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Subject[RFC][PATCH 3/3] CGroups: Add css_tryget()
This patch adds 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>

---
include/linux/cgroup.h | 33 ++++++++++++++++++++++-----
kernel/cgroup.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-----
2 files changed, 80 insertions(+), 11 deletions(-)
Index: hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
===================================================================
--- hierarchy_lock-mmotm-2008-12-09.orig/include/linux/cgroup.h
+++ hierarchy_lock-mmotm-2008-12-09/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 +80,27 @@ static inline void css_get(struct cgroup
if (!test_bit(CSS_ROOT, &css->flags))
atomic_inc(&css->refcnt);
}
+
+/*
+ * 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);
Index: hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
===================================================================
--- hierarchy_lock-mmotm-2008-12-09.orig/kernel/cgroup.c
+++ hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
@@ -2321,7 +2321,7 @@ static void init_cgroup_css(struct cgrou
struct cgroup *cgrp)
{
css->cgroup = cgrp;
- atomic_set(&css->refcnt, 0);
+ atomic_set(&css->refcnt, 1);
css->flags = 0;
if (cgrp == dummytop)
set_bit(CSS_ROOT, &css->flags);
@@ -2453,7 +2453,7 @@ static int cgroup_has_css_refs(struct cg
{
/* Check the reference count on each subsystem. Since we
* already established that there are no tasks in the
- * cgroup, if the css refcount is also 0, then there should
+ * cgroup, if the css refcount is also 1, then there should
* be no outstanding references, so the subsystem is safe to
* destroy. We scan across all subsystems rather than using
* the per-hierarchy linked list of mounted subsystems since
@@ -2474,12 +2474,59 @@ static int cgroup_has_css_refs(struct cg
* matter, since it can only happen if the cgroup
* has been deleted and hence no longer needs the
* release agent to be called anyway. */
- if (css && atomic_read(&css->refcnt))
+ if (css && (atomic_read(&css->refcnt) > 1))
return 1;
}
return 0;
}

+/*
+ * Atomically mark all of the cgroup's CSS objects as
+ * CSS_REMOVED. Return true on success, or false if the cgroup has
+ * busy subsystems. Call with cgroup_mutex held
+ */
+
+static int cgroup_clear_css_refs(struct cgroup *cgrp)
+{
+ struct cgroup_subsys *ss;
+ unsigned long flags;
+ bool failed = false;
+ local_irq_save(flags);
+ for_each_subsys(cgrp->root, ss) {
+ struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
+ int refcnt;
+ do {
+ /* We can only remove a CSS with a refcnt==1 */
+ refcnt = atomic_read(&css->refcnt);
+ if (refcnt > 1) {
+ failed = true;
+ goto done;
+ }
+ BUG_ON(!refcnt);
+ /*
+ * Drop the refcnt to 0 while we check other
+ * subsystems. This will cause any racing
+ * css_tryget() to spin until we set the
+ * CSS_REMOVED bits or abort
+ */
+ } while (atomic_cmpxchg(&css->refcnt, refcnt, 0) != refcnt);
+ }
+ done:
+ for_each_subsys(cgrp->root, ss) {
+ struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
+ if (failed) {
+ /* Restore old refcnt if necessary */
+ if (!atomic_read(&css->refcnt))
+ atomic_set(&css->refcnt, 1);
+ } else {
+ /* Commit the fact that the CSS is removed */
+ set_bit(CSS_REMOVED, &css->flags);
+ }
+ }
+ local_irq_restore(flags);
+ return !failed;
+}
+
static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
struct cgroup *cgrp = dentry->d_fsdata;
@@ -2510,7 +2557,7 @@ static int cgroup_rmdir(struct inode *un

if (atomic_read(&cgrp->count)
|| !list_empty(&cgrp->children)
- || cgroup_has_css_refs(cgrp)) {
+ || !cgroup_clear_css_refs(cgrp)) {
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
@@ -3065,7 +3112,8 @@ void __css_put(struct cgroup_subsys_stat
{
struct cgroup *cgrp = css->cgroup;
rcu_read_lock();
- if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {
+ if ((atomic_dec_return(&css->refcnt) == 1) &&
+ notify_on_release(cgrp)) {
set_bit(CGRP_RELEASABLE, &cgrp->flags);
check_for_release(cgrp);
}
--


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