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

[Li Zefan <lizf@cn.fujitsu.com>]

>>> 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