[Ocfs2-devel] ocfs2 quota qs.

[Tao Ma]

Jan Kara wrote:
> On Wed 29-07-09 17:04:14, Tao Ma wrote:
>   
>> Hi Jan,
>> 	I am now reading quota support in ocfs2 and have some qs. Wish you can 
>> help me clarify it. Thanks.
>>
>> ocfs2_quota_write:
>> 	In the comment, you said that "we know the transaction is already 
>> started", so it should be called within a transaction. But in this 
>> function, we call ocfs2_extend_no_holes and ocfs2_simple_size_update which 
>> will start another transaction. So we can survive with nested transaction 
>> or this scenario will never happen?
>>     
>   The comment is right, we absolutely require a transaction being started
> when when entering ocfs2_quota_write(), otherwise deadlocks could occur.
>   JBD supports nested transactions - when journal_start() is called and
> the current process already has a transaction open, it just returns the
> currently open transaction. So you only have to be sure to initially ask
> for enough credits when opening the transaction - and quota code hopefully
> gets this right.
>   Hmm, but looking at the code again, there's one catch - normally,
> allocator locks nest outside the transaction and in ocfs2_quota_write()
> they are acquired inside the transaction and that can lead to deadlocks.
> Aw, that's going to be nasty to fix... I have to rethink the locking
> logic...
>   Thanks for your inquiry :)
>   
Thank you for the explanation.

Regards,
Tao