Re: [PATCH 1/3] cgroup: remove tasklist_lock from cgroup_attach_proc

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

>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>> index 1042b3c..032139d 100644
>> --- a/kernel/cgroup.c
>> +++ b/kernel/cgroup.c
>> @@ -2102,21 +2102,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
>>  	if (retval)
>>  		goto out_free_group_list;
>>  
>> -	/* prevent changes to the threadgroup list while we take a snapshot. */
>> -	read_lock(&tasklist_lock);
>> -	if (!thread_group_leader(leader)) {
>> -		/*
>> -		 * a race with de_thread from another thread's exec() may strip
>> -		 * us of our leadership, making while_each_thread unsafe to use
>> -		 * on this task. if this happens, there is no choice but to
>> -		 * throw this task away and try again (from cgroup_procs_write);
>> -		 * this is "double-double-toil-and-trouble-check locking".
>> -		 */
>> -		read_unlock(&tasklist_lock);
>> -		retval = -EAGAIN;
>> -		goto out_free_group_list;
>> -	}
>> -
>>  	tsk = leader;
>>  	i = 0;
>>  	do {
>> @@ -2145,7 +2130,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
>>  	group_size = i;
>>  	tset.tc_array = group;
>>  	tset.tc_array_len = group_size;
>> -	read_unlock(&tasklist_lock);
> 
> You still need rcu_read_lock()/rcu_read_unlock() around
> 	do {
> 
> 	} while_each_thread()
> 
> because threadgroup_lock() doesn't lock the part that remove a thread from
> its group on exit.
> 

and inside rcu critical section, you can't call kmalloc(GFP_KERNEL)!!