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authorTejun Heo <tj@kernel.org>2012-11-05 09:16:58 -0800
committerTejun Heo <tj@kernel.org>2012-11-05 09:16:58 -0800
commite93160803ffda2e67d9ff9cacb63bb6868c8398f (patch)
treee3305bb56336c32862f1970882f030caca7f11b2 /kernel
parented95779340b50e362245c81b5dec0d11a1debfa8 (diff)
cgroup: kill CSS_REMOVED
CSS_REMOVED is one of the several contortions which were necessary to support css reference draining on cgroup removal. All css->refcnts which need draining should be deactivated and verified to equal zero atomically w.r.t. css_tryget(). If any one isn't zero, all refcnts needed to be re-activated and css_tryget() shouldn't fail in the process. This was achieved by letting css_tryget() busy-loop until either the refcnt is reactivated (failed removal attempt) or CSS_REMOVED is set (committing to removal). Now that css refcnt draining is no longer used, there's no need for atomic rollback mechanism. css_tryget() simply can look at the reference count and fail if it's deactivated - it's never getting re-activated. This patch removes CSS_REMOVED and updates __css_tryget() to fail if the refcnt is deactivated. As deactivation and removal are a single step now, they no longer need to be protected against css_tryget() happening from irq context. Remove local_irq_disable/enable() from cgroup_rmdir(). Note that this removes css_is_removed() whose only user is VM_BUG_ON() in memcontrol.c. We can replace it with a check on the refcnt but given that the only use case is a debug assert, I think it's better to simply unexport it. v2: Comment updated and explanation on local_irq_disable/enable() added per Michal Hocko. Signed-off-by: Tejun Heo <tj@kernel.org> Reviewed-by: Michal Hocko <mhocko@suse.cz> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Acked-by: Li Zefan <lizefan@huawei.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Balbir Singh <bsingharora@gmail.com>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/cgroup.c31
1 files changed, 12 insertions, 19 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8c605e2bdd1a..c194f9e4fc7b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -170,8 +170,8 @@ struct css_id {
* The css to which this ID points. This pointer is set to valid value
* after cgroup is populated. If cgroup is removed, this will be NULL.
* This pointer is expected to be RCU-safe because destroy()
- * is called after synchronize_rcu(). But for safe use, css_is_removed()
- * css_tryget() should be used for avoiding race.
+ * is called after synchronize_rcu(). But for safe use, css_tryget()
+ * should be used for avoiding race.
*/
struct cgroup_subsys_state __rcu *css;
/*
@@ -4112,8 +4112,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
}
prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);
- local_irq_disable();
-
/* block new css_tryget() by deactivating refcnt */
for_each_subsys(cgrp->root, ss) {
struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
@@ -4123,21 +4121,14 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
}
/*
- * Set REMOVED. All in-progress css_tryget() will be released.
* Put all the base refs. Each css holds an extra reference to the
* cgroup's dentry and cgroup removal proceeds regardless of css
* refs. On the last put of each css, whenever that may be, the
* extra dentry ref is put so that dentry destruction happens only
* after all css's are released.
*/
- for_each_subsys(cgrp->root, ss) {
- struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
-
- set_bit(CSS_REMOVED, &css->flags);
- css_put(css);
- }
-
- local_irq_enable();
+ for_each_subsys(cgrp->root, ss)
+ css_put(cgrp->subsys[ss->subsys_id]);
finish_wait(&cgroup_rmdir_waitq, &wait);
clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
@@ -4861,15 +4852,17 @@ static void check_for_release(struct cgroup *cgrp)
/* Caller must verify that the css is not for root cgroup */
bool __css_tryget(struct cgroup_subsys_state *css)
{
- do {
- int v = css_refcnt(css);
+ while (true) {
+ int t, v;
- if (atomic_cmpxchg(&css->refcnt, v, v + 1) == v)
+ v = css_refcnt(css);
+ t = atomic_cmpxchg(&css->refcnt, v, v + 1);
+ if (likely(t == v))
return true;
+ else if (t < 0)
+ return false;
cpu_relax();
- } while (!test_bit(CSS_REMOVED, &css->flags));
-
- return false;
+ }
}
EXPORT_SYMBOL_GPL(__css_tryget);