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Subject[PATCH 1/2] memcg: cgroup fix rmdir hang
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Now, cgroup has a logic to wait until ready-to-rmdir for avoiding
frequent -EBUSY at rmdir.
(See Commit ec64f51545fffbc4cb968f0cea56341a4b07e85a
cgroup: fix frequent -EBUSY at rmdir.

Nishimura-san reported bad case for waiting and This is a fix to
make it reliable. A thread waiting for thread cannot be waken up
when a refcnt gotten by css_tryget() isn't put immediately.
(Original code assumed css_put() will be called soon.)

memcg has this case and this is a fix for the problem. This adds
retry_rmdir() callback to subsys and check we can sleep or not.

Note: another solution will be adding "rmdir state" to subsys.
But it will be much complicated than this do-enough-check solution.

Changelog v1 -> v2:
- splitted into 2 patches. This just includes retry_rmdir() modification.

Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
Documentation/cgroups/cgroups.txt | 11 +++++++++++
include/linux/cgroup.h | 1 +
kernel/cgroup.c | 20 +++++++++++++++++++-
mm/memcontrol.c | 14 ++++++++++++--
4 files changed, 43 insertions(+), 3 deletions(-)

Index: fix-rmdir-cgroup/include/linux/cgroup.h
===================================================================
--- fix-rmdir-cgroup.orig/include/linux/cgroup.h
+++ fix-rmdir-cgroup/include/linux/cgroup.h
@@ -374,6 +374,7 @@ struct cgroup_subsys {
struct cgroup_subsys_state *(*create)(struct cgroup_subsys *ss,
struct cgroup *cgrp);
int (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
+ int (*retry_rmdir)(struct cgroup_subsys *ss, struct cgroup *cgrp);
void (*destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
int (*can_attach)(struct cgroup_subsys *ss,
struct cgroup *cgrp, struct task_struct *tsk);
Index: fix-rmdir-cgroup/kernel/cgroup.c
===================================================================
--- fix-rmdir-cgroup.orig/kernel/cgroup.c
+++ fix-rmdir-cgroup/kernel/cgroup.c
@@ -636,6 +636,23 @@ static int cgroup_call_pre_destroy(struc
}
return ret;
}
+/*
+ * Call subsys's retry_rmdir() handler. If this returns non-Zero, we retry
+ * rmdir immediately and call pre_destroy again.
+ */
+static int cgroup_check_retry_rmdir(struct cgroup *cgrp)
+{
+ struct cgroup_subsys *ss;
+ int ret = 0;
+
+ for_each_subsys(cgrp->root, ss)
+ if (ss->pre_destroy) {
+ ret = ss->retry_rmdir(ss, cgrp);
+ if (ret)
+ break;
+ }
+ return ret;
+}

static void free_cgroup_rcu(struct rcu_head *obj)
{
@@ -2722,7 +2739,8 @@ again:

if (!cgroup_clear_css_refs(cgrp)) {
mutex_unlock(&cgroup_mutex);
- schedule();
+ if (!cgroup_check_retry_rmdir(cgrp))
+ schedule();
finish_wait(&cgroup_rmdir_waitq, &wait);
clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
if (signal_pending(current))
Index: fix-rmdir-cgroup/mm/memcontrol.c
===================================================================
--- fix-rmdir-cgroup.orig/mm/memcontrol.c
+++ fix-rmdir-cgroup/mm/memcontrol.c
@@ -1457,8 +1457,6 @@ __mem_cgroup_commit_charge_swapin(struct
}
rcu_read_unlock();
}
- /* add this page(page_cgroup) to the LRU we want. */
-
}

void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
@@ -2571,6 +2569,17 @@ static int mem_cgroup_pre_destroy(struct
return mem_cgroup_force_empty(mem, false);
}

+static int mem_cgroup_retry_rmdir(struct cgroup_subsys *ss,
+ struct cgroup *cont)
+{
+ struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+
+ if (res_counter_read_u64(&mem->res, RES_USAGE))
+ return 1;
+ return 0;
+}
+
+
static void mem_cgroup_destroy(struct cgroup_subsys *ss,
struct cgroup *cont)
{
@@ -2610,6 +2619,7 @@ struct cgroup_subsys mem_cgroup_subsys =
.subsys_id = mem_cgroup_subsys_id,
.create = mem_cgroup_create,
.pre_destroy = mem_cgroup_pre_destroy,
+ .retry_rmdir = mem_cgroup_retry_rmdir,
.destroy = mem_cgroup_destroy,
.populate = mem_cgroup_populate,
.attach = mem_cgroup_move_task,
Index: fix-rmdir-cgroup/Documentation/cgroups/cgroups.txt
===================================================================
--- fix-rmdir-cgroup.orig/Documentation/cgroups/cgroups.txt
+++ fix-rmdir-cgroup/Documentation/cgroups/cgroups.txt
@@ -500,6 +500,17 @@ there are not tasks in the cgroup. If pr
rmdir() will fail with it. From this behavior, pre_destroy() can be
called multiple times against a cgroup.

+int retry_rmdir(struct cgroup_subsys *ss, struct cgroup *cgrp);
+
+Called at rmdir right after the kernel finds there are remaining refcnt on
+subsystems after pre_destroy(). When retry_rmdir() returns 0, the caller enter
+sleep and wakes up when css's refcnt goes down to 0 by css_put().
+When this returns 1, the caller doesn't sleep and retry rmdir immediately.
+This is useful when the subsys knows remaining css's refcnt is not temporal
+and to calling pre_destroy() again is proper way to remove that.
+(or proper way to retrun -EBUSY.)
+
+
int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
struct task_struct *task)
(cgroup_mutex held by caller)


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