Re: [PATCH 04/10] Display /proc/stat information per cgroup

From: Glauber Costa
Date: Wed Oct 05 2011 - 08:44:00 EST

On 10/05/2011 04:38 PM, Peter Zijlstra wrote:
On Wed, 2011-10-05 at 16:10 +0400, Glauber Costa wrote:
On 10/05/2011 12:56 PM, Peter Zijlstra wrote:
On Sun, 2011-10-02 at 23:21 +0400, Glauber Costa wrote:
+struct kernel_stat *task_group_kstat(struct task_struct *p)
+ struct task_group *tg;
+ struct kernel_stat *kstat;
+ rcu_read_lock();
+ tg = task_group(p);
+ kstat = tg->cpustat;
+ rcu_read_unlock();
+ return kstat;

Who keeps tg alive and kicking while you poke at its (cpustat) member?

* All calls to this function currently pass current as a parameter
(Okay, maybe it is too generic and we should pass nothing at all, and
grab current within it)
* rcu_read_lock() guarantees that current will exist during this call,
and task_group won't change. (right?)

The thing I worry about is:

A (pid n) B

kstat = task_group_kstat()
echo n> /cgroup/something-else/pid
rmdir /cgroup/group-that-had-A
<timer interrupt>
RCU complete
kfree(tg) etc..

kstat->foo++;<-- *BOOM*

The only way to avoid someone moving you around is by holding some
cgroup lock, task->alloc_lock, task->pi_lock or the rq->lock where task
runs. Alternatively keep rcu_read_lock() around the entire kstat usage.

Well, if I understand it correctly, we'd have to hold the lock around the entire kstat usge as well, right?

Otherwise it can just explode. So rcu does seem the way to go. I do, however, see the problem you are describing.

Maybe we can remove the rcu_read_lock() call and replace with a rcu validation. Then patch all callers. (Including of course the current users of kstat_cpu() all over). What do you think?

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