Re: [PATCH] memcg: fix oops in oom handling

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

KAMEZAWA Hiroyuki wrote:
> On Mon, 14 Apr 2008 14:52:00 +0800
> Li Zefan <lizf@cn.fujitsu.com> wrote:
>> It's reproducable in a x86_64 box, but doesn't happen in x86_32.
>>
>> This is because tsk->sighand is not guarded by RCU, so we have to
>> hold tasklist_lock, just as what out_of_memory() does.
>>
> 
> Good catch! and patch seems good.
> 
> Paul, I have one confirmation. Lock hierarchy of
> 	cgroup_lock()
> 	->	read_lock(&tasklist_lock)
> 
> is ok ? (I think this is ok.)
> 

I think so, because we used to have this in cgroup:

mutex_lock(&cgroup_mutex);
  -> attach_task_by_pid()
       -> read_lock(&tasklist_lock)

But afte some time, some uses of tasklist_lock is replaced by rcu.

> Thanks,
> -Kame
> 
>> Signed-off-by: Li Zefan <lizf@cn.fujitsu>
>> ---
>>  mm/oom_kill.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>> index f255eda..beb592f 100644
>> --- a/mm/oom_kill.c
>> +++ b/mm/oom_kill.c
>> @@ -423,7 +423,7 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
>>  	struct task_struct *p;
>>  
>>  	cgroup_lock();
>> -	rcu_read_lock();
>> +	read_lock(&tasklist_lock);
>>  retry:
>>  	p = select_bad_process(&points, mem);
>>  	if (PTR_ERR(p) == -1UL)
>> @@ -436,7 +436,7 @@ retry:
>>  				"Memory cgroup out of memory"))
>>  		goto retry;
>>  out:
>> -	rcu_read_unlock();
>> +	read_unlock(&tasklist_lock);
>>  	cgroup_unlock();
>>  }
>>  #endif
>> -- 1.5.4.rc3 
>>
>>
>>
>>
> 

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