linux-kernel - Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct

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Message-ID: <20090317073649.GH3314@in.ibm.com>
Date:	Tue, 17 Mar 2009 13:06:49 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in
	cpuacct_charge() safe when rcupreempt is used.

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@...ux.vnet.ibm.com>
---
 kernel/sched.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9891,6 +9891,13 @@ static void cpuacct_charge(struct task_s
 		return;
 
 	cpu = task_cpu(tsk);
+
+	/*
+	 * preemption is already disabled here, but to be safe with
+	 * rcupreempt, take rcu_read_lock(). This protects ca and
+	 * hence the hierarchy walk.
+	 */
+	rcu_read_lock();
 	ca = task_ca(tsk);
 
 	do {
@@ -9898,6 +9905,7 @@ 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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