Re: [-mm][PATCH 4/4] Add memrlimit controller accounting and control (v5)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Vivek Goyal wrote:
> On Wed, May 21, 2008 at 09:00:12PM +0530, Balbir Singh wrote:
> 
> [..]
>> +static void memrlimit_cgroup_move_task(struct cgroup_subsys *ss,
>> +					struct cgroup *cgrp,
>> +					struct cgroup *old_cgrp,
>> +					struct task_struct *p)
>> +{
>> +	struct mm_struct *mm;
>> +	struct memrlimit_cgroup *memrcg, *old_memrcg;
>> +
>> +	mm = get_task_mm(p);
>> +	if (mm == NULL)
>> +		return;
>> +
>> +	/*
>> +	 * Hold mmap_sem, so that total_vm does not change underneath us
>> +	 */
>> +	down_read(&mm->mmap_sem);
>> +
>> +	rcu_read_lock();
>> +	if (p != rcu_dereference(mm->owner))
>> +		goto out;
>> +
> 
> Hi Balbir,
> 
> How does rcu help here? We are not dereferencing mm->owner. So even if
> task_struct it was pointing to goes away, should not be a problem.
> 

Yes, you are right, since we already have information about the cgroup and new
cgroup, mm->owner's exit should not really cause a problem

> OTOH, while updating the mm->owner in mmm_update_next_owner(), we
> are not using rcu_assing_pointer() and synchronize_rcu()/call_rcu(). Is
> this the right usage if mm->owner is rcu protected?
> 

Yes, you are correct - I'll send out updates on top of this one.

> Thanks
> Vivek
> 
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-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL

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