[Cluster-devel] Why does dlm_lock function fails when downconvert a dlm lock?

[David Teigland]

On Fri, Aug 13, 2021 at 02:49:04PM +0800, Gang He wrote:
> Hi David,
> 
> On 2021/8/13 1:45, David Teigland wrote:
> > On Thu, Aug 12, 2021 at 01:44:53PM +0800, Gang He wrote:
> > > In fact, I can reproduce this problem stably.
> > > I want to know if this error happen is by our expectation? since there is
> > > not any extreme pressure test.
> > > Second, how should we handle these error cases? call dlm_lock function
> > > again? maybe the function will fails again, that will lead to kernel
> > > soft-lockup after multiple re-tries.
> > 
> > What's probably happening is that ocfs2 calls dlm_unlock(CANCEL) to cancel
> > an in-progress dlm_lock() request.  Before the cancel completes (or the
> > original request completes), ocfs2 calls dlm_lock() again on the same
> > resource.  This dlm_lock() returns -EBUSY because the previous request has
> > not completed, either normally or by cancellation.  This is expected.
> These dlm_lock and dlm_unlock are invoked in the same node, or the different
> nodes?

different

> > A couple options to try: wait for the original request to complete
> > (normally or by cancellation) before calling dlm_lock() again, or retry
> > dlm_lock() on -EBUSY.
> If I retry dlm_lock() repeatedly, I just wonder if this will lead to kernel
> soft lockup or waste lots of CPU.

I'm not aware of other code doing this, so I can't tell you with certainty.
It would depend largely on the implementation in the caller.

> If dlm_lock() function returns -EAGAIN, how should we handle this case?
> retry it repeatedly?

Again, this is a question more about the implementation of the calling
code and what it wants to do.  EAGAIN is specifically related to the
DLM_LKF_NOQUEUE flag.