Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk incpuacct_charge() safe when rcupreempt is used.

[Bharata B Rao]

On Thu, Mar 19, 2009 at 10:20:21AM +0100, Peter Zijlstra wrote:
> On Tue, 2009-03-17 at 13:06 +0530, Bharata B Rao wrote:
> > On Tue, Mar 17, 2009 at 02:28:11PM +0800, Li Zefan wrote:
> > > Bharata B Rao wrote:
> > > > cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when
> > > > 	rcupreempt is used.
> > > > 
> > > > cpuacct_charge() obtains task's ca and does a hierarchy walk upwards.
> > > > This can race with the task's movement between cgroups. This race
> > > > can cause an access to freed ca pointer in cpuacct_charge(). This will not
> > > 
> > > Actually it can also end up access invalid tsk->cgroups. ;)
> > > 
> > > get tsk->cgroups (cg)
> > >                          (move tsk to another cgroup) or (tsk exiting)
> > >                          -> kfree(tsk->cgroups)
> > > get cg->subsys[..]
> > 
> > Ok :) Here is the patch again with updated description.
> > 
> > cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when
> > 	rcupreempt is used.
> > 
> > cpuacct_charge() obtains task's ca and does a hierarchy walk upwards.
> > This can race with the task's movement between cgroups. This race
> > can cause an access to freed ca pointer in cpuacct_charge() or access
> > to invalid cgroups pointer of the task. This will not happen with rcu or
> > tree rcu as cpuacct_charge() is called with preemption disabled. However if
> > rcupreempt is used, the race is seen. Thanks to Li Zefan for explaining this.
> > 
> > Fix this race by explicitly protecting ca and the hierarchy walk with
> > rcu_read_lock().
> > 
> > Signed-off-by: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx>
> 
> I would ditch the comment, it doesn't add anything.
> 
> The simple rule is: if you want RCU-safe, use rcu_read_lock().
> preempt/irq disable isn't sufficient -- hasn't been for a long long
> while.
> 
> After that,
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> 

Ok. Removed the comment. Here is the updated patch.

cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when
	rcupreempt is used.

cpuacct_charge() obtains task's ca and does a hierarchy walk upwards.
This can race with the task's movement between cgroups. This race
can cause an access to freed ca pointer in cpuacct_charge() or access
to invalid cgroups pointer of the task. This will not happen with rcu or
tree rcu as cpuacct_charge() is called with preemption disabled. However if
rcupreempt is used, the race is seen. Thanks to Li Zefan for explaining this.

Fix this race by explicitly protecting ca and the hierarchy walk with
rcu_read_lock().

Signed-off-by: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Acked-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
Tested-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
---
 kernel/sched.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9894,6 +9894,8 @@ static void cpuacct_charge(struct task_s
 		return;
 
 	cpu = task_cpu(tsk);
+
+	rcu_read_lock();
 	ca = task_ca(tsk);
 
 	do {
@@ -9901,6 +9903,7 @@ static void cpuacct_charge(struct task_s
 		*cpuusage += cputime;
 		ca = ca->parent;
 	} while (ca);
+	rcu_read_unlock();
 }
 
 struct cgroup_subsys cpuacct_subsys = {
--
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