On 2012/11/15 3:04, Tejun Heo wrote:
> Hello, Li.
>
> On Wed, Nov 14, 2012 at 11:20:47AM +0800, Li Zefan wrote:
>>> It also removes the need to check whether cgroup->dentry is %NULL in
>>> cgroup_path. If a cgroup is visible, its dentry is guaranteed to be
>>> visible too.
>>
>> I'm afraid this isn't safe.
>>
>> The cgroup is visible to a controller soon after ss->create(), and then
>> the controller might call cgroup_path() while cgrp->dentry is still NULL.
>
> Hmmm... I can't find any case where ss->create() is calling
> cgroup_path(). Do you remember which one that was?
>
I meant cgroup_create() may race with cgroup_path(). We used to have
this race:
(cpu0) (cpu1)
mkdir cat /proc/sched_debug
cgroup_create()
cpu_cgroup_create()
register_fair_sched_group()
list_add_rcu(...)
print_cfs_stats()
for_each_leaf_cfs_rq()
print_cfs_rq()
cgroup_path()
css->cgroup = cgroup;
cgroup->dentry = dentry;
In this case, cgroup_path() might access NULL cgroup or NULL dentry.
But I've looked the current scheduler code, no race anymore, because now the
cfs_rq won't be added to the gloal list until there's task in the cgroup.
(in the old time it was added to the list when creating a cgroup)
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