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

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

Yes, I noticed the lockres_set_pending and lockres_clear_pending doesn't 
exist in 1.4 code.
But 1.4 code did have the problem, that when lock a new lockres, 
lockres->l_action = OCFS2_AST_ATTACH,
and l_flags |= OCFS2_LOCK_BUSY ,and release the spin lock before ast was 
queued. Also there is no pending status for
downconvert thread to check and quit before the BUG().


On 02/22/2012 02:04 AM, Sunil Mushran wrote:
> Moreover what is lockres_clear_pending doing in 1.4. That code
> is not meant for 1.4. It fixes a problem associated with fsdlm.
> It was left out of 1.4 for a reason.
>
> Meaning this bug was introduced by the patch that introduced this
> one in 1.4.
>
> 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.
>>
>>
>