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

[Sunil Mushran <sunil.mushran@oracle.com>]

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.
>
>