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Subject[PATCH] sched: fix another race when reading /proc/sched_debug
I fixed an oops with the following commit:

| commit 24eb089950ce44603b30a3145a2c8520e2b55bb1
| Author: Li Zefan <lizf@cn.fujitsu.com>
| Date: Thu Nov 6 12:53:32 2008 -0800
|
| cgroups: fix invalid cgrp->dentry before cgroup has been completely removed
|
| This fixes an oops when reading /proc/sched_debug.

The above commit fixed a race that reading /proc/sched_debug may access
NULL cgrp->dentry if a cgroup is being removed.

But I found there's another different race, in that reading sched_debug
may access a cgroup which hasn't been completed created, and thus
dereference NULL cgrp->dentry!

cgroup_create()
cpu_cgroup_create()
register_fair_sched_group()
list_add_rcu(...)
print_cfs_stats()
for_each_leaf_cfs_rq()
print_cfs_rq()
cgroup_path()
cgroup->dentry = dentry;

task_group is added to the global list before the cgroup has been created
completely, if at this time print_cfs_stats() is called, it will access
the half-created cgroup.

This patch fixes the bug by holding cgroup_lock() to wait the cgroup to
be created completely before calling cgroup_path().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
The patch is based on linus's git tree, and should go into 2.6.28,
but it conflicts with the cleanup patch in sched/core:
0a0db8f5c9d4bbb9bbfcc2b6cb6bce2d0ef4d73d

---
kernel/sched_debug.c | 10 ++++++++--
1 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index 26ed8e3..01abf5b 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -127,8 +127,11 @@ 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) {
+ cgroup_lock();
cgroup_path(cgroup, path, sizeof(path));
+ cgroup_unlock();
+ }

SEQ_printf(m, "\ncfs_rq[%d]:%s\n", cpu, path);
#else
@@ -181,8 +184,11 @@ 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) {
+ cgroup_lock();
cgroup_path(cgroup, path, sizeof(path));
+ cgroup_unlock();
+ }

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

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