Re: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue()

[Li Zefan <lizf@cn.fujitsu.com>]

>>>>>> With RCU correctness on, We see following warning. This patch fixes it.
>>>>> This is in initialization code, so that there cannot be any concurrent
>>>>> updates, correct?  If so, looks good.
>>>>>
>>>> I think theoritically two instances of cfq_init_queue() can be running
>>>> in parallel (for two different devices), and they both can call
>>>> blkiocg_add_blkio_group(). But then we use a spin lock to protect
>>>> blkio_cgroup.
>>>>
>>>> spin_lock_irqsave(&blkcg->lock, flags);
>>>>
>>>> So I guess two parallel updates should be fine.
>>> OK, in that case, would it be possible add this spinlock to the condition
>>> checked by css_id()'s rcu_dereference_check()?
>> Hi Paul,
>>
>> I think adding these spinlock to condition checked might become little
>> messy. And the reason being that this lock is subsystem (controller)
>> specific and maintained by controller. Now if any controller implements
>> a lock and we add that lock in css_id() rcu_dereference_check(), it will
>> look ugly.
>>
>> So probably a better way is to make sure that css_id() is always called
>> under rcu read lock so that we don't hit this warning?
> 
> As long as holding rcu_read_lock() prevents css_id() from the usual
> problems such as access memory that was concurrently freed, yes.
> 

blkiocg_add_blkio_group() also calls cgroup_path(), which also needs to
be called within rcu_read_lock, so I think Vivek's patch is better than
the one you posted in another mail thread.