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cgroup: kill CSS_REMOVED

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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>
This commit is contained in:
Tejun Heo 2012-11-05 09:16:58 -08:00
parent ed95779340
commit e93160803f
3 changed files with 15 additions and 29 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

6
include/linux/cgroup.h
View file

@ -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

31
kernel/cgroup.c
View file

@ -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);

7
mm/memcontrol.c
View file

@ -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)
{