Re: [RFC] [PATCH 1/5] cgroups: revamp subsys array

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

Fix "To" and "Cc"..

Li Zefan wrote:
> Ben Blum wrote:
>> On Tue, Dec 08, 2009 at 03:38:43PM +0800, Li Zefan wrote:
>>>> @@ -1291,6 +1324,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
>>>>  	struct cgroupfs_root *new_root;
>>>>  
>>>>  	/* First find the desired set of subsystems */
>>>> +	down_read(&subsys_mutex);
>>> Hmm.. this can lead to deadlock. sget() returns success with sb->s_umount
>>> held, so here we have:
>>>
>>> 	down_read(&subsys_mutex);
>>>
>>> 	down_write(&sb->s_umount);
>>>
>>> On the other hand, sb->s_umount is held before calling kill_sb(),
>>> so when umounting we have:
>>>
>>> 	down_write(&sb->s_umount);
>>>
>>> 	down_read(&subsys_mutex);
>> Unless I'm gravely mistaken, you can't have deadlock on an rwsem when
>> it's being taken for reading in both cases? You would have to have at
>> least one of the cases being down_write.
>>
> 
> lockdep will warn on this..
> 
> And it can really lead to deadlock, though not so obivously:
> 
>   thread 1       thread 2        thread 3
> -------------------------------------------
> | read(A)        write(B)
> |
> |                                write(A)
> |
> |                read(A)
> |
> | write(B)
> |
> 
> t3 is waiting for t1 to release the lock, then t2 tries to
> acquire A lock to read, but it has to wait because of t3,
> and t1 has to wait t2.
> 
> Note: a read lock has to wait if a write lock is already
> waiting for the lock.
> 
>> In fairness to readability, perhaps subsys_mutex should instead be
>> subsys_rwsem? It seemed to me to be that calling it "mutex" was
>> conventional anyway.
>>