[Ocfs2-devel] [PATCH] ocfs2: fix __ocfs2_cluster_lock() dead lock

[Joel Becker]

On Tue, Jan 12, 2010 at 11:58:35AM +0800, Wengang Wang wrote:
> On 10-01-11 17:59, Joel Becker wrote:
> > Date: Wed, 6 Jan 2010 16:34:44 +0800
> > Subject: [PATCH] ocfs2: fix __ocfs2_cluster_lock() livelock
> > 
> > There is livelock possibility in __ocfs2_cluster_lock().  Here's what
> > happens:
> > 
> > 1) node A(the lock owner) is doing an up-convert(UC, PR->EX). it gets
> > the lock but doesn't check BUSY again because the level is right.
> > 
> > 2) node B requested an UC on the same lock resource. since node A has an
> > EX on the lockres, a bast is issued and OCFS2_LOCK_BLOCKED flag is set
> > to the lockres meanting that a DC is should be done. the DC asks for
> > agreement of node A to release EX.  The downconvert starts because BUSY
> > is cleared by the ast.
> > 
> > 3) the UC on node A runs into the check of OCFS2_LOCK_BLOCKED:
> > 
> >         if (lockres->l_flags & OCFS2_LOCK_BLOCKED &&
> >             !ocfs2_may_continue_on_blocked_lock(lockres, level)) {
> > 
> > analysis:
> > the BLOCKED flag is set in 2), and the UC can't continue to get the lock
> > (ocfs2_may_continue_on_blocked_lock() returns false), so it waits on the
> > DC to finish.
> > 
> > 4) The DC finishes, and BLOCKED is cleared.  The UC on node A starts over
> >    getting the UC, now from NL->EX.  It asks node B for the lock.
> 
> I am wondering why DC can finish.
> although BUSY is cleared. I think the check_downconvert() should return
> false since we have not done whatever we want to do. and then the DC should
> be requeued.

	DC thread only waits on PENDING and holders.  If BUSY, it will
cancel the upconvert.  If not BUSY, it will schedule a downconvert.
There's nothing stopping the downconvert, in other words.

> #don't hate me if I am wrong/stupid :)

	Won't hate you at all.  And you're not stupid even if you are
wrong.  It's not easy code.  Heck, you may be right and I might be wrong
in the end.  Remember, you found the bug in the first place!

Joel

-- 

Life's Little Instruction Book #450

	"Don't be afraid to say, 'I need help.'"

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127