[Ocfs2-devel] Race condition between OCFS2 downconvert thread and ocfs2 cluster lock.

[Xiaowei.hu <xiaowei.hu@oracle.com>]

Hi Sunil,

I mean it execute in this way:

nodeA ocfs2_dlm_lock() and released the res spin lock,here A doesn't 
hold spin locks,
then it start to execute the proxy ast handler , process bast request 
from nodeB,
then dlmthread flushed the bast, after this node A start to queue its 
ast in ocfs2_dlm_lock() function.

Thanks,
Xiaowei
On 02/22/2012 01:48 AM, Sunil Mushran wrote:
> > bast queued and flushed,before the ast was queued
>
> Unlikely with o2dlm. dlmthread always sends ASTs before BASTs.
>
> Can you recreate the entire lockres? A full dump may yield more
> information.
>
> Sunil
>
> On 02/20/2012 10:12 PM, xiaowei.hu at oracle.com wrote:
>> I am trying to fix bug13611997,CT's machine run into BUG in ocfs2dc 
>> thread, BUG_ON(lockres->l_action != OCFS2_AST_CONVERT&&  
>> lockres->l_action != OCFS2_AST_DOWNCONVERT); I analysized the vmcore 
>> , the lockres->l_action = OCFS2_AST_ATTACH and l_flags=326(which 
>> means 
>> OCFS2_LOCK_BUSY|OCFS2_LOCK_BLOCKED|OCFS2_LOCK_INITIALIZED|OCFS2_LOCK_QUEUED), 
>> after compared with the code , this status could be only possible 
>> during ocfs2_cluster_lock,here is the race situation:
>>
>> NodeA                                NodeB
>> ocfs2_cluster_lock on a new lockres M
>> spin_lock_irqsave(&lockres->l_lock, flags);
>> gen = lockres_set_pending(lockres);
>> lockres->l_action = OCFS2_AST_ATTACH;
>> lockres_or_flags(lockres, OCFS2_LOCK_BUSY);
>> spin_unlock_irqrestore(&lockres->l_lock, flags);
>>
>> ocfs2_dlm_lock() finished and returned.
>> **and lockres_clear_pending(lockres, gen, osb);
>>                             request a lock on the same lockres M
>>                             It's blocked by nodeA, and a ast proxy 
>> was send to A
>>
>> bast queued and flushed,before the ast was queued
>> then the ocfs2dc was scheduled
>> there is a chance to execute this code path:
>> ocfs2_downconvert_thread()
>> ocfs2_downconvert_thread_do_work()
>> ocfs2_blocking_ast()
>> ocfs2_process_blocked_lock()
>> ocfs2_unblock_lock()
>>     spin_lock_irqsave(&lockres->l_lock, flags);
>>     if (lockres->l_flags&  OCFS2_LOCK_BUSY)
>>         ret = ocfs2_prepare_cancel_convert(osb, lockres);
>>         BUG_ON(lockres->l_action != OCFS2_AST_CONVERT&&
>>                     lockres->l_action != OCFS2_AST_DOWNCONVERT);
>>         here trigger the BUG()
>>
>> Solution:
>> One possible solution for this is to remove the lockres_clear_pending 
>> marked by 2 stars, and left this clear work to the ast function.In 
>> this way could make sure the bast function wait for ast , let it 
>> clear OCFS2_LOCK_BUSY and set OCFS2_LOCK_ATTACHED first, before enter 
>> downconvert process.
>>
>>
>