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

[Gang He]

Hello List,

I am using kernel 5.13.4 (some old version kernels have the same problem).
When node A acquired a dlm (EX) lock, node B tried to get the dlm lock, node A got a BAST message,
then node A downcoverted the dlm lock to NL, dlm_lock function failed with the error -16.
The function failure did not always happen, but in some case, I could encounter this failure. 
Why does dlm_lock function fails when downconvert a dlm lock? there are any documents for describe these error cases?
If the code ignores dlm_lock return error from node A, node B will not get the dlm lock permanently.
How should we handle such situation? call dlm_lock function to downconvert the dlm lock again?

Thanks
Gang