Re: [cgroup or VFS ?] INFO: possible recursive locking detected

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

Al Viro wrote:
> On Thu, Jan 08, 2009 at 11:45:43AM +0800, Li Zefan wrote:
>> Hi Al Viro,
>>
>> I hacked into the kernel with the patch below (I think It's ok for me
>> to comment out bdev->bd_mount_sem for testing):
> 
>> And ran 2 threads:
>> 	for ((; ;))  # thread 1
>> 	{
>> 		mount -t ext3 /dev/sda9 /mnt1
>> 		umount /mnt1
>> 	}
>>
>> 	for ((; ;))  # thread 2
>> 	{
>> 		mount -t ext3 /dev/sda9 /mnt2
>> 		umount /mnt2
>> 	}
>>
>> And I got the same lockdep warning immediately, so I think it's
>> VFS's issue.
> 
> It's a lockdep issue, actually.  It _is_ a false positive; we could get rid

Yes, I believe it's a false positive when I looked into this issue.

> of that if we took destroy_super(s); just before grab_super(), but I really
> do not believe that there's any point.
> 
> Frankly, I'd rather see if there's any way to teach lockdep that this instance
> of lock is getting initialized into "one writer" state and that yes, we know
> that it's not visible to anyone, so doing that is safe, TYVM, even though
> we are under spinlock.  Then take that sucker to just before set().
> 

It would be nice if we can do this way..

> In any case, I really do not believe that it might have anything to do with
> the WARN_ON() from another thread...
> 

agreed. I don't think they are related, and that's why I sent 2 different reports.