Messages in this thread | | | Date | Wed, 18 Mar 2009 15:06:24 +0530 | From | Balbir Singh <> | Subject | Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used. |
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* Bharata B Rao <bharata@linux.vnet.ibm.com> [2009-03-18 08:48:32]:
> On Tue, Mar 17, 2009 at 06:42:51PM +0530, Balbir Singh wrote: > > * Bharata B Rao <bharata@linux.vnet.ibm.com> [2009-03-17 13:06:49]: > > > > > 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(). > > > > > > > Looks good and works very well (except for the batch issue that you > > pointed out, it takes up to batch values before updates are seen). > > > > I'd like to get the patches in -tip and see the results, I would > > recommend using percpu_counter_sum() while reading the data as an > > enhancement to this patch. If user space does not overwhelm with a lot > > of reads, sum would work out better. > > > > > > Tested-by: Balbir Singh <balbir@linux.vnet.ibm.com> > > Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com> > > So I guess this ack is not for this patch but for the per-cgroup > stime/utime cpuacct controller statistics patch. >
Yes.. for both these patches actually. Thanks for pointing it out though.
-- Balbir
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