Messages in this thread |  | | | Subject | Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used. | | From | Peter Zijlstra <> | | Date | Thu, 19 Mar 2009 10:20:21 +0100 |
| |
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@linux.vnet.ibm.com>
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@chello.nl>
> --- > 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 = {
|  |