[Ocfs2-devel] [PATCH] ocfs2: fix __ocfs2_cluster_lock() dead lock
Joel Becker
Joel.Becker at oracle.com
Wed Jan 6 18:00:06 PST 2010
On Wed, Jan 06, 2010 at 04:34:44PM +0800, Wengang Wang wrote:
> there is deadlock possibility in __ocfs2_cluster_lock().
> the case is like following.
>
> #in time order
> 1) node A(the lock owner) is doing an up-convert(UC, PR->EX). it got the lock
> but didn't check BUSY flag again.
>
> 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. and for now node A refuses that(still in process of getting that
> lock:) ).
>
> 3) the UC on node A runs into the phrase of checking OCFS2_LOCK_BLOCKED flag and
> if it can continue without waiting for any DC. code is:
>
> 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 the DC to finish.
>
> so both UC and DC are waiting for each other --a dead lock.
1) Do you have a test case? Have you seen this happen, or are you just
postulating from reading the code?
2) It sure looks like you're right.
3) I hate our locking spaghetti!
Joel
--
"I don't know anything about music. In my line you don't have
to."
- Elvis Presley
Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127
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