Commit a18b83b7 authored by Bharata B Rao's avatar Bharata B Rao Committed by Ingo Molnar

cpuacct: make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used -v2

Impact: fix cgroups race under rcu-preempt

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().

Changes for v2:

 - Update patch descrition (as per Li Zefan's review comments).

 - Remove comments in cpuacct_charge() which explained why rcu_read_lock()
   was needed (as per Peter Zijlstra's review comments).
Signed-off-by: default avatarBharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
Tested-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 2f850181
...@@ -9654,12 +9654,17 @@ static void cpuacct_charge(struct task_struct *tsk, u64 cputime) ...@@ -9654,12 +9654,17 @@ static void cpuacct_charge(struct task_struct *tsk, u64 cputime)
return; return;
cpu = task_cpu(tsk); cpu = task_cpu(tsk);
rcu_read_lock();
ca = task_ca(tsk); ca = task_ca(tsk);
for (; ca; ca = ca->parent) { for (; ca; ca = ca->parent) {
u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu); u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
*cpuusage += cputime; *cpuusage += cputime;
} }
rcu_read_unlock();
} }
struct cgroup_subsys cpuacct_subsys = { struct cgroup_subsys cpuacct_subsys = {
......
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