Re: [PATCH] sched: fix another race when reading /proc/sched_debug

Li Zefan
2008-12-15T09:52:45Z

>>> Can't we detect a dead task-group and skip those instead of adding this
>>> global lock?
>>>
>> I tried it, but I don't think it's feasable, without lock syncronization:
>>
>> | print_cfs_rq()
>> | check task_group is dead
>> cgroup_diput() |
>> .. |
>> mark task_group as dead |
>> .. |
>> kfree(cgrp) |
>> | call cgroup_path()
>
> rcu free cgrp
>

I got your point, thanks.

Another way is use css_tryget(), and thus can avoid touching cgroup.c and adding
synchronize_rcu(). css_tryget() is proposed by Kamezawa but I think won't be
available until 2.6.29.

Anyway, here is the fix. I'll post a complete version with changelog when we
agree on how to fix it.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
kernel/cgroup.c | 6 ++++++
kernel/sched_debug.c | 17 +++++++++++++++--
2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fe00b3b..3c54d1b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -624,6 +624,12 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
* created the cgroup */
deactivate_super(cgrp->root->sb);

+ /*
+ * Some subsystems (cpu cgroup) might still be able to
+ * accessing the cgroup in rcu section.
+ */
+ synchronize_rcu();
+
kfree(cgrp);
}
iput(inode);
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index 26ed8e3..174c072 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -127,8 +127,14 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
if (tg)
cgroup = tg->css.cgroup;

- if (cgroup)
+ if (cgroup) {
+ /*
+ * This task_group is dead or we race with cgroup creating.
+ */
+ if (cgroup_is_removed(cgroup) || !cgroup->dentry)
+ return;
cgroup_path(cgroup, path, sizeof(path));
+ }

SEQ_printf(m, "\ncfs_rq[%d]:%s\n", cpu, path);
#else
@@ -181,8 +187,15 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
if (tg)
cgroup = tg->css.cgroup;

- if (cgroup)
+ if (cgroup) {
+ /*
+ * This task_group is dead or we race with cgroup creating.
+ */
+ if (cgroup_is_removed(cgroup) || !cgroup->dentry)
+ return;
+
cgroup_path(cgroup, path, sizeof(path));
+ }

SEQ_printf(m, "\nrt_rq[%d]:%s\n", cpu, path);
#else
--
1.5.4.rc3