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From: Paul E. McKenney on 30 Apr 2010 20:30
From: Li Zefan <lizf(a)cn.fujitsu.com>

With CONFIG_PROVE_RCU=y, a warning can be triggered:

$ cat /proc/sched_debug

....
kernel/cgroup.c:1649 invoked rcu_dereference_check() without protection!
....

Both cgroup_path() and task_group() should be called with either
rcu_read_lock or cgroup_mutex held.

The rcu_dereference_check() does include cgroup_lock_is_held(), so we
know that this lock is not held. Therefore, in a CONFIG_PREEMPT kernel,
to say nothing of a CONFIG_PREEMPT_RT kernel, the original code could
have ended up copying a string out of the freelist.

This patch inserts RCU read-side primitives needed to prevent this
scenario.

Signed-off-by: Li Zefan <lizf(a)cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck(a)linux.vnet.ibm.com>
---
kernel/sched_debug.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index 9b49db1..19be00b 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -114,7 +114,9 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
{
char path[64];

+ rcu_read_lock();
cgroup_path(task_group(p)->css.cgroup, path, sizeof(path));
+ rcu_read_unlock();
SEQ_printf(m, " %s", path);
}
#endif
--
1.7.0

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Next: [PATCH tip/core/urgent 10/10] vfs: fix RCU-lockdep false positive due to /proc access