[PATCH 1/3] cgroup: fix pre_destroy and semantics of css->refcnt

From: KAMEZAWA Hiroyuki
Date: Mon Dec 01 2008 - 01:03:16 EST

Now, final check of refcnt is done after pre_destroy(), so rmdir() can fail
after pre_destroy().
memcg set mem->obsolete to be 1 at pre_destroy and this is buggy..

Several ways to fix this can be considered. This is an idea.

Fortunately, the user of css_get()/css_put() is only memcg, now.
And it seems assumption on css_ref in cgroup.c is a bit complicated.
I'd like to reuse it.
This patch changes this css->refcnt usage and action as following
- css->refcnt is initialized to 1.

- after pre_destroy, before destroy(), try to drop css->refcnt to 0.

- css_tryget() is added. This only success when css->refcnt > 0.

- css_is_removed() is added. This checks css->refcnt == 0 and means
this cgroup is under destroy() or not.

- css_put() is changed not to call notify_on_release().
From documentation, notify_on_release() is called when there is no
tasks/children in cgroup. On implementation, notify_on_release is
not called if css->refcnt > 0.
This is problematic. memcg has css->refcnt by each page even when
there are no tasks. release handler will be never called.
But, now, rmdir()/pre_destroy() of memcg works well and checking
checking css->ref is not (and shouldn't be) necessary for notifying.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxx>


include/linux/cgroup.h | 21 +++++++++++++++++--
kernel/cgroup.c | 53 +++++++++++++++++++++++++++++++++++--------------
mm/memcontrol.c | 40 +++++++++++++++++++++++++-----------
3 files changed, 85 insertions(+), 29 deletions(-)

Index: mmotm-2.6.28-Nov29/include/linux/cgroup.h
===================================================================
--- mmotm-2.6.28-Nov29.orig/include/linux/cgroup.h
+++ mmotm-2.6.28-Nov29/include/linux/cgroup.h
@@ -54,7 +54,9 @@ struct cgroup_subsys_state {

/* State maintained by the cgroup system to allow
* subsystems to be "busy". Should be accessed via css_get()
- * and css_put() */
+ * and css_put(). If this value is 0, css is now under removal and
+ * destroy() will be called soon. (and there is no roll-back.)
+ */

atomic_t refcnt;

@@ -86,7 +88,22 @@ extern void __css_put(struct cgroup_subs
static inline void css_put(struct cgroup_subsys_state *css)
{
if (!test_bit(CSS_ROOT, &css->flags))
- __css_put(css);
+ atomic_dec(&css->refcnt);
+}
+
+/* returns not-zero if success */
+static inline int css_tryget(struct cgroup_subsys_state *css)
+{
+ if (!test_bit(CSS_ROOT, &css->flags))
+ return atomic_inc_not_zero(&css->refcnt);
+ return 1;
+}
+
+static inline bool css_under_removal(struct cgroup_subsys_state *css)
+{
+ if (test_bit(CSS_ROOT, &css->flags))
+ return false;
+ return atomic_read(&css->refcnt) == 0;
}

/* bits in struct cgroup flags field */
Index: mmotm-2.6.28-Nov29/kernel/cgroup.c
===================================================================
--- mmotm-2.6.28-Nov29.orig/kernel/cgroup.c
+++ mmotm-2.6.28-Nov29/kernel/cgroup.c
@@ -589,6 +589,32 @@ static void cgroup_call_pre_destroy(stru
return;
}

+/*
+ * Try to set all subsys's refcnt to be 0.
+ * css->refcnt==0 means this subsys will be destroy()'d.
+ */
+static bool cgroup_set_subsys_removed(struct cgroup *cgrp)
+{
+ struct cgroup_subsys *ss;
+ struct cgroup_subsys_state *css, *tmp;
+
+ for_each_subsys(cgrp->root, ss) {
+ css = cgrp->subsys[ss->subsys_id];
+ if (!atomic_dec_and_test(&css->refcnt))
+ goto rollback;
+ }
+ return true;
+rollback:
+ for_each_subsys(cgrp->root, ss) {
+ tmp = cgrp->subsys[ss->subsys_id];
+ atomic_inc(&tmp->refcnt);
+ if (tmp == css)
+ break;
+ }
+ return false;
+}
+
+
static void cgroup_diput(struct dentry *dentry, struct inode *inode)
{
/* is dentry a directory ? if so, kfree() associated cgroup */
@@ -2310,7 +2336,7 @@ 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);
@@ -2438,7 +2464,7 @@ 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;
@@ -2465,7 +2491,8 @@ static int cgroup_rmdir(struct inode *un

/*
* Call pre_destroy handlers of subsys. Notify subsystems
- * that rmdir() request comes.
+ * that rmdir() request comes. pre_destroy() is expected to drop all
+ * extra refcnt to css. (css->refcnt == 1)
*/
cgroup_call_pre_destroy(cgrp);

@@ -2479,8 +2506,15 @@ static int cgroup_rmdir(struct inode *un
return -EBUSY;
}

+ /* last check ! */
+ if (!cgroup_set_subsys_removed(cgrp)) {
+ mutex_unlock(&cgroup_mutex);
+ return -EBUSY;
+ }
+
spin_lock(&release_list_lock);
set_bit(CGRP_REMOVED, &cgrp->flags);
+
if (!list_empty(&cgrp->release_list))
list_del(&cgrp->release_list);
spin_unlock(&release_list_lock);
@@ -3003,7 +3037,7 @@ static void check_for_release(struct cgr
/* All of these checks rely on RCU to keep the cgroup
* structure alive */
if (cgroup_is_releasable(cgrp) && !atomic_read(&cgrp->count)
- && list_empty(&cgrp->children) && !cgroup_has_css_refs(cgrp)) {
+ && list_empty(&cgrp->children)) {
/* Control Group is currently removeable. If it's not
* already queued for a userspace notification, queue
* it now */
@@ -3020,17 +3054,6 @@ static void check_for_release(struct cgr
}
}

-void __css_put(struct cgroup_subsys_state *css)
-{
- struct cgroup *cgrp = css->cgroup;
- rcu_read_lock();
- if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {
- set_bit(CGRP_RELEASABLE, &cgrp->flags);
- check_for_release(cgrp);
- }
- rcu_read_unlock();
-}
-
/*
* Notify userspace when a cgroup is released, by running the
* configured release agent with the name of the cgroup (path
Index: mmotm-2.6.28-Nov29/mm/memcontrol.c
===================================================================
--- mmotm-2.6.28-Nov29.orig/mm/memcontrol.c
+++ mmotm-2.6.28-Nov29/mm/memcontrol.c
@@ -154,7 +154,6 @@ struct mem_cgroup {
*/
bool use_hierarchy;
unsigned long last_oom_jiffies;
- int obsolete;
atomic_t refcnt;
/*
* statistics. This must be placed at the end of memcg.
@@ -540,8 +539,14 @@ mem_cgroup_get_first_node(struct mem_cgr
{
struct cgroup *cgroup;
struct mem_cgroup *ret;
- bool obsolete = (root_mem->last_scanned_child &&
- root_mem->last_scanned_child->obsolete);
+ struct mem_cgroup *last_scan = root_mem->last_scanned_child;
+ bool obsolete = false;
+
+ if (last_scan) {
+ if (css_under_removal(&last_scan->css))
+ obsolete = true;
+ } else
+ obsolete = true;

/*
* Scan all children under the mem_cgroup mem
@@ -598,7 +603,7 @@ static int mem_cgroup_hierarchical_recla
next_mem = mem_cgroup_get_first_node(root_mem);

while (next_mem != root_mem) {
- if (next_mem->obsolete) {
+ if (css_under_removal(&next_mem->css)) {
mem_cgroup_put(next_mem);
cgroup_lock();
next_mem = mem_cgroup_get_first_node(root_mem);
@@ -985,6 +990,7 @@ int mem_cgroup_try_charge_swapin(struct
{
struct mem_cgroup *mem;
swp_entry_t ent;
+ int ret;

if (mem_cgroup_disabled())
return 0;
@@ -1003,10 +1009,18 @@ int mem_cgroup_try_charge_swapin(struct
ent.val = page_private(page);

mem = lookup_swap_cgroup(ent);
- if (!mem || mem->obsolete)
+ /*
+ * Because we can't assume "mem" is alive now, use tryget() and
+ * drop extra count later
+ */
+ if (!mem || !css_tryget(&mem->css))
goto charge_cur_mm;
*ptr = mem;
- return __mem_cgroup_try_charge(NULL, mask, ptr, true);
+ ret = __mem_cgroup_try_charge(NULL, mask, ptr, true);
+ /* drop extra count */
+ css_put(&mem->css);
+
+ return ret;
charge_cur_mm:
if (unlikely(!mm))
mm = &init_mm;
@@ -1037,14 +1051,16 @@ int mem_cgroup_cache_charge_swapin(struc
ent.val = page_private(page);
if (do_swap_account) {
mem = lookup_swap_cgroup(ent);
- if (mem && mem->obsolete)
+ if (mem && !css_tryget(&mem->css))
mem = NULL;
if (mem)
mm = NULL;
}
ret = mem_cgroup_charge_common(page, mm, mask,
MEM_CGROUP_CHARGE_TYPE_SHMEM, mem);
-
+ /* drop extra ref */
+ if (mem)
+ css_put(&mem->css);
if (!ret && do_swap_account) {
/* avoid double counting */
mem = swap_cgroup_record(ent, NULL);
@@ -1886,8 +1902,8 @@ static struct mem_cgroup *mem_cgroup_all
* the number of reference from swap_cgroup and free mem_cgroup when
* it goes down to 0.
*
- * When mem_cgroup is destroyed, mem->obsolete will be set to 0 and
- * entry which points to this memcg will be ignore at swapin.
+ * When mem_cgroup is destroyed, css_under_removal() is true and entry which
+ * points to this memcg will be ignore at swapin.
*
* Removal of cgroup itself succeeds regardless of refs from swap.
*/
@@ -1917,7 +1933,7 @@ static void mem_cgroup_get(struct mem_cg
static void mem_cgroup_put(struct mem_cgroup *mem)
{
if (atomic_dec_and_test(&mem->refcnt)) {
- if (!mem->obsolete)
+ if (!css_under_removal(&mem->css))
return;
mem_cgroup_free(mem);
}
@@ -1980,7 +1996,7 @@ static void mem_cgroup_pre_destroy(struc
struct cgroup *cont)
{
struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
- mem->obsolete = 1;
+ /* dentry's mutex makes this safe. */
mem_cgroup_force_empty(mem, false);
}


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