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>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 02e09c0..a309804 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -85,7 +85,6 @@ 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 */
 };
 
 /* Caller must verify that the css is not for root cgroup */
@@ -108,11 +107,6 @@ static inline void css_get(struct cgroup_subsys_state *css)
                __css_get(css, 1);
 }
 
-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

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8c605e2..c194f9e 100644 (file)
--- 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);
 

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5a1d584..37c3566 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2343,7 +2343,6 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 again:
        if (*ptr) { /* css should be a valid one */
                memcg = *ptr;
-               VM_BUG_ON(css_is_removed(&memcg->css));
                if (mem_cgroup_is_root(memcg))
                        goto done;
                if (nr_pages == 1 && consume_stock(memcg))
@@ -2483,9 +2482,9 @@ static void __mem_cgroup_cancel_local_charge(struct mem_cgroup *memcg,
 
 /*
  * A helper function to get mem_cgroup from ID. must be called under
- * rcu_read_lock(). The caller must check css_is_removed() or some if
- * it's concern. (dropping refcnt from swap can be called against removed
- * memcg.)
+ * rcu_read_lock().  The caller is responsible for calling css_tryget if
+ * the mem_cgroup is used for charging. (dropping refcnt from swap can be
+ * called against removed memcg.)
  */
 static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
 {